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# Detect loop in a linked list

• Difficulty Level : Easy
• Last Updated : 28 Oct, 2022

Given a linked list, check if the linked list has a loop or not. The below diagram shows a linked list with a loop.

## Detect loop in a linked list using Hashing:

The idea is to insert the nodes in the hashmap and whenever a node is encountered that is already present in the hashmap then return true.

Follow the steps below to solve the problem:

• Traverse the list individually and keep putting the node addresses in a Hash Table.
• At any point, if NULL is reached then return false
• If the next of the current nodes points to any of the previously stored nodes in  Hash then return true.

Below is the implementation of the above approach:

## C++

 `// C++ program to detect loop in a linked list``#include ``using` `namespace` `std;` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``struct` `Node* new_node = ``new` `Node;` `    ``/* put in the data  */``    ``new_node->data = new_data;` `    ``/* link the old list off the new node */``    ``new_node->next = (*head_ref);` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `// Returns true if there is a loop in linked list``// else returns false.``bool` `detectLoop(``struct` `Node* h)``{``    ``unordered_set s;``    ``while` `(h != NULL) {``        ``// If this node is already present``        ``// in hashmap it means there is a cycle``        ``// (Because you will be encountering the``        ``// node for the second time).``        ``if` `(s.find(h) != s.end())``            ``return` `true``;` `        ``// If we are seeing the node for``        ``// the first time, insert it in hash``        ``s.insert(h);` `        ``h = h->next;``    ``}` `    ``return` `false``;``}` `/* Driver program to test above function*/``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next = head;` `    ``if` `(detectLoop(head))``        ``cout << ``"Loop Found"``;``    ``else``        ``cout << ``"No Loop"``;` `    ``return` `0;``}``// This code is contributed by Geetanjali`

## Java

 `// Java program to detect loop in a linked list``import` `java.util.*;` `public` `class` `LinkedList {` `    ``static` `Node head; ``// head of list` `    ``/* Linked list Node*/``    ``static` `class` `Node {``        ``int` `data;``        ``Node next;``        ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Inserts a new Node at front of the list. */``    ``static` `public` `void` `push(``int` `new_data)``    ``{``        ``/* 1 & 2: Allocate the Node &``                  ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``// Returns true if there is a loop in linked``    ``// list else returns false.``    ``static` `boolean` `detectLoop(Node h)``    ``{``        ``HashSet s = ``new` `HashSet();``        ``while` `(h != ``null``) {``            ``// If we have already has this node``            ``// in hashmap it means their is a cycle``            ``// (Because you we encountering the``            ``// node second time).``            ``if` `(s.contains(h))``                ``return` `true``;` `            ``// If we are seeing the node for``            ``// the first time, insert it in hash``            ``s.add(h);` `            ``h = h.next;``        ``}` `        ``return` `false``;``    ``}` `    ``/* Driver program to test above function */``    ``public` `static` `void` `main(String[] args)``    ``{``        ``LinkedList llist = ``new` `LinkedList();` `        ``llist.push(``20``);``        ``llist.push(``4``);``        ``llist.push(``15``);``        ``llist.push(``10``);` `        ``/*Create loop for testing */``        ``llist.head.next.next.next.next = llist.head;` `        ``if` `(detectLoop(head))``            ``System.out.println(``"Loop Found"``);``        ``else``            ``System.out.println(``"No Loop"``);``    ``}``}` `// This code is contributed by Arnav Kr. Mandal.`

## Python3

 `# Python3 program to detect loop``# in the linked list` `# Node class``class` `Node:` `    ``# Constructor to initialize``    ``# the node object``    ``def` `__init__(``self``, data):``        ``self``.data ``=` `data``        ``self``.``next` `=` `None`  `class` `LinkedList:` `    ``# Function to initialize head``    ``def` `__init__(``self``):``        ``self``.head ``=` `None` `    ``# Function to insert a new``    ``# node at the beginning``    ``def` `push(``self``, new_data):``        ``new_node ``=` `Node(new_data)``        ``new_node.``next` `=` `self``.head``        ``self``.head ``=` `new_node` `    ``# Utility function to print it``    ``# the linked LinkedList``    ``def` `printList(``self``):``        ``temp ``=` `self``.head``        ``while``(temp):``            ``print``(temp.data, end``=``" "``)``            ``temp ``=` `temp.``next` `    ``def` `detectLoop(``self``):``        ``s ``=` `set``()``        ``temp ``=` `self``.head``        ``while` `(temp):` `            ``# If we already have``            ``# this node in hashmap it``            ``# means there is a cycle``            ``# (Because we are encountering``            ``# the node second time).``            ``if` `(temp ``in` `s):``                ``return` `True` `            ``# If we are seeing the node for``            ``# the first time, insert it in hash``            ``s.add(temp)` `            ``temp ``=` `temp.``next` `        ``return` `False`  `# Driver program for testing``llist ``=` `LinkedList()``llist.push(``20``)``llist.push(``4``)``llist.push(``15``)``llist.push(``10``)` `# Create a loop for testing``llist.head.``next``.``next``.``next``.``next` `=` `llist.head` `if``(llist.detectLoop()):``    ``print``(``"Loop Found"``)``else``:``    ``print``(``"No Loop "``)` `# This code is contributed by Gitanjali.`

## C#

 `// C# program to detect loop in a linked list``using` `System;``using` `System.Collections.Generic;` `class` `LinkedList {` `    ``// head of list``    ``public` `Node head;` `    ``/* Linked list Node*/``    ``public` `class` `Node {``        ``public` `int` `data;``        ``public` `Node next;``        ``public` `Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Inserts a new Node at front of the list. */``    ``public` `void` `push(``int` `new_data)``    ``{``        ``/* 1 & 2: Allocate the Node &``                ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``// Returns true if there is a loop in linked``    ``// list else returns false.``    ``public` `static` `bool` `detectLoop(Node h)``    ``{``        ``HashSet s = ``new` `HashSet();``        ``while` `(h != ``null``) {``            ``// If we have already has this node``            ``// in hashmap it means their is a cycle``            ``// (Because you we encountering the``            ``// node second time).``            ``if` `(s.Contains(h))``                ``return` `true``;` `            ``// If we are seeing the node for``            ``// the first time, insert it in hash``            ``s.Add(h);` `            ``h = h.next;``        ``}` `        ``return` `false``;``    ``}` `    ``/* Driver code*/``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``LinkedList llist = ``new` `LinkedList();` `        ``llist.push(20);``        ``llist.push(4);``        ``llist.push(15);``        ``llist.push(10);` `        ``/*Create loop for testing */``        ``llist.head.next.next.next.next = llist.head;` `        ``if` `(detectLoop(llist.head))``            ``Console.WriteLine(``"Loop Found"``);``        ``else``            ``Console.WriteLine(``"No Loop"``);``    ``}``}` `// This code has been contributed by 29AjayKumar`

## Javascript

 ``

Output

`Loop Found`

Time complexity: O(N), Only one traversal of the loop is needed.
Auxiliary Space: O(N), N is the space required to store the value in the hashmap.

## Detect loop in a linked list by Modification In Node Structure:

The idea is to modify the node structure by adding flag in it and mark the flag whenever visit the node.

Follow the steps below to solve the problem:

• Have a visited flag with each node.
• Traverse the linked list and keep marking visited nodes.
• If you see a visited node again then there is a loop.

Below is the implementation of the above approach.

## C++

 `// C++ program to detect loop in a linked list``#include ``using` `namespace` `std;` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``    ``int` `flag;``};` `void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``struct` `Node* new_node = ``new` `Node;` `    ``/* put in the data */``    ``new_node->data = new_data;` `    ``new_node->flag = 0;` `    ``/* link the old list off the new node */``    ``new_node->next = (*head_ref);` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `// Returns true if there is a loop in linked list``// else returns false.``bool` `detectLoop(``struct` `Node* h)``{``    ``while` `(h != NULL) {``        ``// If this node is already traverse``        ``// it means there is a cycle``        ``// (Because you we encountering the``        ``// node for the second time).``        ``if` `(h->flag == 1)``            ``return` `true``;` `        ``// If we are seeing the node for``        ``// the first time, mark its flag as 1``        ``h->flag = 1;` `        ``h = h->next;``    ``}` `    ``return` `false``;``}` `/* Driver program to test above function*/``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next = head;` `    ``if` `(detectLoop(head))``        ``cout << ``"Loop Found"``;``    ``else``        ``cout << ``"No Loop"``;` `    ``return` `0;``}``// This code is contributed by Geetanjali`

## C

 `// C program to detect loop in a linked list``#include ``#include ``#include ` `/* Link list node */``typedef` `struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``    ``int` `flag;``} Node;` `void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``struct` `Node* new_node = (Node*)``malloc``(``sizeof``(Node));` `    ``/* put in the data */``    ``new_node->data = new_data;` `    ``new_node->flag = 0;` `    ``/* link the old list off the new node */``    ``new_node->next = (*head_ref);` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `// Returns true if there is a loop in linked list``// else returns false.``bool` `detectLoop(``struct` `Node* h)``{``    ``while` `(h != NULL) {``        ``// If this node is already traverse``        ``// it means there is a cycle``        ``// (Because you we encountering the``        ``// node for the second time).``        ``if` `(h->flag == 1)``            ``return` `true``;` `        ``// If we are seeing the node for``        ``// the first time, mark its flag as 1``        ``h->flag = 1;` `        ``h = h->next;``    ``}` `    ``return` `false``;``}` `/* Driver program to test above function*/``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next = head;` `    ``if` `(detectLoop(head))``        ``printf``(``"Loop Found"``);``    ``else``        ``printf``(``"No Loop"``);` `    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## Java

 `// Java program to detect loop in a linked list``import` `java.util.*;` `class` `GFG {` `    ``// Link list node``    ``static` `class` `Node {``        ``int` `data;``        ``Node next;``        ``int` `flag;``    ``};` `    ``static` `Node push(Node head_ref, ``int` `new_data)``    ``{` `        ``// Allocate node``        ``Node new_node = ``new` `Node();` `        ``// Put in the data``        ``new_node.data = new_data;` `        ``new_node.flag = ``0``;` `        ``// Link the old list off the new node``        ``new_node.next = head_ref;` `        ``// Move the head to point to the new node``        ``head_ref = new_node;``        ``return` `head_ref;``    ``}` `    ``// Returns true if there is a loop in linked``    ``// list else returns false.``    ``static` `boolean` `detectLoop(Node h)``    ``{``        ``while` `(h != ``null``) {` `            ``// If this node is already traverse``            ``// it means there is a cycle``            ``// (Because you we encountering the``            ``// node for the second time).``            ``if` `(h.flag == ``1``)``                ``return` `true``;` `            ``// If we are seeing the node for``            ``// the first time, mark its flag as 1``            ``h.flag = ``1``;` `            ``h = h.next;``        ``}``        ``return` `false``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{` `        ``// Start with the empty list``        ``Node head = ``null``;` `        ``head = push(head, ``20``);``        ``head = push(head, ``4``);``        ``head = push(head, ``15``);``        ``head = push(head, ``10``);` `        ``// Create a loop for testing``        ``head.next.next.next.next = head;` `        ``if` `(detectLoop(head))``            ``System.out.print(``"Loop Found"``);``        ``else``            ``System.out.print(``"No Loop"``);``    ``}``}` `// This code is contributed by Rajput-Ji`

## Python3

 `# Python3 program to detect loop in a linked list` `''' Link list node '''`  `class` `Node:` `    ``def` `__init__(``self``):``        ``self``.data ``=` `0``        ``self``.``next` `=` `None``        ``self``.flag ``=` `0`  `def` `push(head_ref, new_data):``    ``''' allocate node '''``    ``new_node ``=` `Node()` `    ``''' put in the data '''``    ``new_node.data ``=` `new_data` `    ``new_node.flag ``=` `0` `    ``''' link the old list off the new node '''``    ``new_node.``next` `=` `(head_ref)` `    ``''' move the head to point to the new node '''``    ``(head_ref) ``=` `new_node``    ``return` `head_ref` `# Returns true if there is a loop in linked list``# else returns false.`  `def` `detectLoop(h):` `    ``while` `(h !``=` `None``):``        ``# If this node is already traverse``        ``# it means there is a cycle``        ``# (Because you we encountering the``        ``# node for the second time).``        ``if` `(h.flag ``=``=` `1``):``            ``return` `True` `        ``# If we are seeing the node for``        ``# the first time, mark its flag as 1``        ``h.flag ``=` `1``        ``h ``=` `h.``next``    ``return` `False`  `''' Driver program to test above function'''``if` `__name__ ``=``=` `'__main__'``:` `    ``''' Start with the empty list '''``    ``head ``=` `None` `    ``head ``=` `push(head, ``20``)``    ``head ``=` `push(head, ``4``)``    ``head ``=` `push(head, ``15``)``    ``head ``=` `push(head, ``10``)` `    ``''' Create a loop for testing '''``    ``head.``next``.``next``.``next``.``next` `=` `head` `    ``if` `(detectLoop(head)):``        ``print``(``"Loop Found"``)``    ``else``:``        ``print``(``"No Loop"``)` `# This code is contributed by rutvik_56`

## C#

 `// C# program to detect loop in a linked list``using` `System;` `class` `GFG {` `    ``// Link list node``    ``class` `Node {``        ``public` `int` `data;``        ``public` `Node next;``        ``public` `int` `flag;``    ``};` `    ``static` `Node push(Node head_ref, ``int` `new_data)``    ``{` `        ``// Allocate node``        ``Node new_node = ``new` `Node();` `        ``// Put in the data``        ``new_node.data = new_data;` `        ``new_node.flag = 0;` `        ``// Link the old list off the new node``        ``new_node.next = head_ref;` `        ``// Move the head to point to the new node``        ``head_ref = new_node;``        ``return` `head_ref;``    ``}` `    ``// Returns true if there is a loop in linked``    ``// list else returns false.``    ``static` `bool` `detectLoop(Node h)``    ``{``        ``while` `(h != ``null``) {` `            ``// If this node is already traverse``            ``// it means there is a cycle``            ``// (Because you we encountering the``            ``// node for the second time).``            ``if` `(h.flag == 1)``                ``return` `true``;` `            ``// If we are seeing the node for``            ``// the first time, mark its flag as 1``            ``h.flag = 1;` `            ``h = h.next;``        ``}``        ``return` `false``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(``string``[] args)``    ``{` `        ``// Start with the empty list``        ``Node head = ``null``;` `        ``head = push(head, 20);``        ``head = push(head, 4);``        ``head = push(head, 15);``        ``head = push(head, 10);` `        ``// Create a loop for testing``        ``head.next.next.next.next = head;` `        ``if` `(detectLoop(head))``            ``Console.Write(``"Loop Found"``);``        ``else``            ``Console.Write(``"No Loop"``);``    ``}``}` `// This code is contributed by pratham76`

## Javascript

 ``

Output

`Loop Found`

Time complexity: O(N), Only one traversal of the loop is needed.
Auxiliary Space: O(1)

## Detect loop in a linked list using Floyd’s Cycle-Finding Algorithm:

This algorithm is used to find a loop in a linked list. It uses two pointers one moving twice as fast as the other one. The faster one is called the faster pointer and the other one is called the slow pointer.

Follow the steps below to solve the problem:

• Traverse linked list using two pointers.
• Move one pointer(slow_p) by one and another pointer(fast_p) by two.
• If these pointers meet at the same node then there is a loop. If pointers do not meet then the linked list doesn’t have a loop.

Illustration:

The below image shows how the detect loop function works in the code:

Implementation of Floyd’s Cycle-Finding Algorithm:

## C++

 `// C++ program to detect loop in a linked list``#include ``using` `namespace` `std;` `/* Link list node */``class` `Node {``public``:``    ``int` `data;``    ``Node* next;``};` `void` `push(Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``Node* new_node = ``new` `Node();` `    ``/* put in the data */``    ``new_node->data = new_data;` `    ``/* link the old list off the new node */``    ``new_node->next = (*head_ref);` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `int` `detectLoop(Node* list)``{``    ``Node *slow_p = list, *fast_p = list;` `    ``while` `(slow_p && fast_p && fast_p->next) {``        ``slow_p = slow_p->next;``        ``fast_p = fast_p->next->next;``        ``if` `(slow_p == fast_p) {``            ``return` `1;``        ``}``    ``}``    ``return` `0;``}` `/* Driver code*/``int` `main()``{``    ``/* Start with the empty list */``    ``Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next = head;``    ``if` `(detectLoop(head))``        ``cout << ``"Loop Found"``;``    ``else``        ``cout << ``"No Loop"``;``    ``return` `0;``}` `// This is code is contributed by rathbhupendra`

## C

 `// C program to detect loop in a linked list``#include ``#include ` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``struct` `Node* new_node``        ``= (``struct` `Node*)``malloc``(``sizeof``(``struct` `Node));` `    ``/* put in the data  */``    ``new_node->data = new_data;` `    ``/* link the old list off the new node */``    ``new_node->next = (*head_ref);` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `int` `detectLoop(``struct` `Node* list)``{``    ``struct` `Node *slow_p = list, *fast_p = list;` `    ``while` `(slow_p && fast_p && fast_p->next) {``        ``slow_p = slow_p->next;``        ``fast_p = fast_p->next->next;``        ``if` `(slow_p == fast_p) {``            ``return` `1;``        ``}``    ``}``    ``return` `0;``}` `/* Driver program to test above function*/``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next = head;` `    ``if` `(detectLoop(head))``        ``printf``(``"Loop Found"``);``    ``else``        ``printf``(``"No Loop"``);``    ``return` `0;``}`

## Java

 `// Java program to detect loop in a linked list` `import` `java.io.*;` `class` `LinkedList {``    ``Node head; ``// head of list` `    ``/* Linked list Node*/``    ``class` `Node {``        ``int` `data;``        ``Node next;``        ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Inserts a new Node at front of the list. */``    ``public` `void` `push(``int` `new_data)``    ``{``        ``/* 1 & 2: Allocate the Node &``                ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``void` `detectLoop()``    ``{``        ``Node slow_p = head, fast_p = head;``        ``int` `flag = ``0``;``        ``while` `(slow_p != ``null` `&& fast_p != ``null``               ``&& fast_p.next != ``null``) {``            ``slow_p = slow_p.next;``            ``fast_p = fast_p.next.next;``            ``if` `(slow_p == fast_p) {``                ``flag = ``1``;``                ``break``;``            ``}``        ``}``        ``if` `(flag == ``1``)``            ``System.out.println(``"Loop Found"``);``        ``else``            ``System.out.println(``"No Loop"``);``    ``}` `    ``/* Driver program to test above functions */``    ``public` `static` `void` `main(String args[])``    ``{``        ``LinkedList llist = ``new` `LinkedList();` `        ``llist.push(``20``);``        ``llist.push(``4``);``        ``llist.push(``15``);``        ``llist.push(``10``);` `        ``/*Create loop for testing */``        ``llist.head.next.next.next.next = llist.head;` `        ``llist.detectLoop();``    ``}``}``/* This code is contributed by Rajat Mishra. */`

## Python

 `# Python program to detect loop in the linked list` `# Node class`  `class` `Node:` `    ``# Constructor to initialize the node object``    ``def` `__init__(``self``, data):``        ``self``.data ``=` `data``        ``self``.``next` `=` `None`  `class` `LinkedList:` `    ``# Function to initialize head``    ``def` `__init__(``self``):``        ``self``.head ``=` `None` `    ``# Function to insert a new node at the beginning``    ``def` `push(``self``, new_data):``        ``new_node ``=` `Node(new_data)``        ``new_node.``next` `=` `self``.head``        ``self``.head ``=` `new_node` `    ``# Utility function to print it the linked LinkedList``    ``def` `printList(``self``):``        ``temp ``=` `self``.head``        ``while``(temp):``            ``print` `temp.data,``            ``temp ``=` `temp.``next` `    ``def` `detectLoop(``self``):``        ``slow_p ``=` `self``.head``        ``fast_p ``=` `self``.head``        ``while``(slow_p ``and` `fast_p ``and` `fast_p.``next``):``            ``slow_p ``=` `slow_p.``next``            ``fast_p ``=` `fast_p.``next``.``next``            ``if` `slow_p ``=``=` `fast_p:``                ``return` `1``        ``return` `0`  `# Driver program for testing``llist ``=` `LinkedList()``llist.push(``20``)``llist.push(``4``)``llist.push(``15``)``llist.push(``10``)` `# Create a loop for testing``llist.head.``next``.``next``.``next``.``next` `=` `llist.head``if``(llist.detectLoop()):``    ``print` `"Loop Found"``else``:``    ``print` `"No Loop"` `# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`

## C#

 `// C# program to detect loop in a linked list``using` `System;` `public` `class` `LinkedList {``    ``Node head; ``// head of list` `    ``/* Linked list Node*/``    ``public` `class` `Node {``        ``public` `int` `data;``        ``public` `Node next;``        ``public` `Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Inserts a new Node at front of the list. */``    ``public` `void` `push(``int` `new_data)``    ``{``        ``/* 1 & 2: Allocate the Node &``                ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``Boolean detectLoop()``    ``{``        ``Node slow_p = head, fast_p = head;``        ``while` `(slow_p != ``null` `&& fast_p != ``null``               ``&& fast_p.next != ``null``) {``            ``slow_p = slow_p.next;``            ``fast_p = fast_p.next.next;``            ``if` `(slow_p == fast_p) {``                ``return` `true``;``            ``}``        ``}``        ``return` `false``;``    ``}` `    ``/* Driver code */``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``LinkedList llist = ``new` `LinkedList();` `        ``llist.push(20);``        ``llist.push(4);``        ``llist.push(15);``        ``llist.push(10);``        ``/*Create loop for testing */``        ``llist.head.next.next.next.next = llist.head;` `        ``Boolean found = llist.detectLoop();``        ``if` `(found) {``            ``Console.WriteLine(``"Loop Found"``);``        ``}``        ``else` `{``            ``Console.WriteLine(``"No Loop"``);``        ``}``    ``}``}` `// This code is contributed by Princi Singh`

## Javascript

 ``

Output

`Loop Found`

Time complexity: O(N), Only one traversal of the loop is needed.
Auxiliary Space: O(1).

## Detect loop in a linked list by Marking visited nodes without modifying Node structure:

The idea is to point the current node of the linked list to a node which is created. Whenever a node’s next is pointing to that node it means loop is there.

Follow the steps below to solve the problem:

• A temporary node is created.
• The next pointer of each node that is traversed is made to point to this temporary node.
• This way we are using the next pointer of a node as a flag to indicate whether the node has been traversed or not.
• Every node is checked to see if the next is pointing to a temporary node or not.
• In the case of the first node of the loop, the second time we traverse it this condition will be true, hence we find that loop exists.
• If we come across a node that points to null then the loop doesnâ€™t exist.

Below is the implementation of the above approach:

## C++

 `// C++ program to return first node of loop``#include ``using` `namespace` `std;` `struct` `Node {``    ``int` `key;``    ``struct` `Node* next;``};` `Node* newNode(``int` `key)``{``    ``Node* temp = ``new` `Node;``    ``temp->key = key;``    ``temp->next = NULL;``    ``return` `temp;``}` `// A utility function to print a linked list``void` `printList(Node* head)``{``    ``while` `(head != NULL) {``        ``cout << head->key << ``" "``;``        ``head = head->next;``    ``}``    ``cout << endl;``}` `// Function to detect first node of loop``// in a linked list that may contain loop``bool` `detectLoop(Node* head)``{``    ``// Create a temporary node``    ``Node* temp = ``new` `Node;``    ``while` `(head != NULL) {``        ``// This condition is for the case when there is no``        ``// loop``        ``if` `(head->next == NULL)``            ``return` `false``;``        ``// Check if next is already pointing to temp``        ``if` `(head->next == temp)``            ``return` `true``;``        ``// Store the pointer to the next node``        ``// in order to get to it in the next step``        ``Node* next = head->next;``        ``// Make next point to temp``        ``head->next = temp;``        ``// Get to the next node in the list``        ``head = next;``    ``}``    ``return` `false``;``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(1);``    ``head->next = newNode(2);``    ``head->next->next = newNode(3);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(5);``    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head->next->next->next->next->next = head->next->next;``    ``bool` `found = detectLoop(head);``    ``if` `(found)``        ``cout << ``"Loop Found"``;``    ``else``        ``cout << ``"No Loop"``;``    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## C

 `// C++ program to return first node of loop``#include ``#include ``#include ` `typedef` `struct` `Node {``    ``int` `key;``    ``struct` `Node* next;``} Node;` `Node* newNode(``int` `key)``{``    ``Node* temp = (Node*)``malloc``(``sizeof``(Node));``    ``temp->key = key;``    ``temp->next = NULL;``    ``return` `temp;``}` `// A utility function to print a linked list``void` `printList(Node* head)``{``    ``while` `(head != NULL) {``        ``printf``(``"%d "``, head->key);``        ``head = head->next;``    ``}``    ``printf``(``"\n"``);``}` `// Function to detect first node of loop``// in a linked list that may contain loop``bool` `detectLoop(Node* head)``{``    ``// Create a temporary node``    ``Node* temp = (Node*)``malloc``(``sizeof``(Node));``    ``while` `(head != NULL) {``        ``// This condition is for the case when there is no``        ``// loop``        ``if` `(head->next == NULL)``            ``return` `false``;``        ``// Check if next is already pointing to temp``        ``if` `(head->next == temp)``            ``return` `true``;``        ``// Store the pointer to the next node``        ``// in order to get to it in the next step``        ``Node* next = head->next;``        ``// Make next point to temp``        ``head->next = temp;``        ``// Get to the next node in the list``        ``head = next;``    ``}``    ``return` `false``;``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(1);``    ``head->next = newNode(2);``    ``head->next->next = newNode(3);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(5);``    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head->next->next->next->next->next = head->next->next;``    ``bool` `found = detectLoop(head);``    ``if` `(found)``        ``printf``(``"Loop Found"``);``    ``else``        ``printf``(``"No Loop"``);``    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## Java

 `// Java program to return first node of loop``public` `class` `GFG {` `    ``static` `class` `Node {``        ``int` `key;``        ``Node next;``    ``};` `    ``static` `Node newNode(``int` `key)``    ``{``        ``Node temp = ``new` `Node();``        ``temp.key = key;``        ``temp.next = ``null``;``        ``return` `temp;``    ``}` `    ``// A utility function to print a linked list``    ``static` `void` `printList(Node head)``    ``{``        ``while` `(head != ``null``) {``            ``System.out.print(head.key + ``" "``);``            ``head = head.next;``        ``}``        ``System.out.println();``    ``}` `    ``// Function to detect first node of loop``    ``// in a linked list that may contain loop``    ``static` `boolean` `detectLoop(Node head)``    ``{` `        ``// Create a temporary node``        ``Node temp = ``new` `Node();``        ``while` `(head != ``null``) {` `            ``// This condition is for the case``            ``// when there is no loop``            ``if` `(head.next == ``null``) {``                ``return` `false``;``            ``}` `            ``// Check if next is already``            ``// pointing to temp``            ``if` `(head.next == temp) {``                ``return` `true``;``            ``}` `            ``// Store the pointer to the next node``            ``// in order to get to it in the next step``            ``Node next = head.next;` `            ``// Make next point to temp``            ``head.next = temp;` `            ``// Get to the next node in the list``            ``head = next;``        ``}` `        ``return` `false``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``        ``Node head = newNode(``1``);``        ``head.next = newNode(``2``);``        ``head.next.next = newNode(``3``);``        ``head.next.next.next = newNode(``4``);``        ``head.next.next.next.next = newNode(``5``);` `        ``// Create a loop for testing(5 is pointing to 3) /``        ``head.next.next.next.next.next = head.next.next;` `        ``boolean` `found = detectLoop(head);``        ``if` `(found)``            ``System.out.println(``"Loop Found"``);``        ``else``            ``System.out.println(``"No Loop"``);``    ``}``};` `// This code is contributed by Arnab Kundu`

## Python3

 `# Python3 program to return first node of loop` `# A binary tree node has data, pointer to``# left child and a pointer to right child``# Helper function that allocates a new node``# with the given data and None left and``# right pointers`  `class` `newNode:``    ``def` `__init__(``self``, key):``        ``self``.key ``=` `key``        ``self``.left ``=` `None``        ``self``.right ``=` `None` `# A utility function to print a linked list`  `def` `printList(head):``    ``while` `(head !``=` `None``):``        ``print``(head.key, end``=``" "``)``        ``head ``=` `head.``next` `    ``print``()` `# Function to detect first node of loop``# in a linked list that may contain loop`  `def` `detectLoop(head):` `    ``# Create a temporary node``    ``temp ``=` `""``    ``while` `(head !``=` `None``):` `        ``# This condition is for the case``        ``# when there is no loop``        ``if` `(head.``next` `=``=` `None``):``            ``return` `False` `        ``# Check if next is already``        ``# pointing to temp``        ``if` `(head.``next` `=``=` `temp):``            ``return` `True` `        ``# Store the pointer to the next node``        ``# in order to get to it in the next step``        ``next` `=` `head.``next` `        ``# Make next point to temp``        ``head.``next` `=` `temp` `        ``# Get to the next node in the list``        ``head ``=` `next` `    ``return` `False`  `# Driver Code``head ``=` `newNode(``1``)``head.``next` `=` `newNode(``2``)``head.``next``.``next` `=` `newNode(``3``)``head.``next``.``next``.``next` `=` `newNode(``4``)``head.``next``.``next``.``next``.``next` `=` `newNode(``5``)` `# Create a loop for testing(5 is pointing to 3)``head.``next``.``next``.``next``.``next``.``next` `=` `head.``next``.``next` `found ``=` `detectLoop(head)``if` `(found):``    ``print``(``"Loop Found"``)``else``:``    ``print``(``"No Loop"``)` `# This code is contributed by SHUBHAMSINGH10`

## C#

 `// C# program to return first node of loop``using` `System;``public` `class` `GFG {` `    ``public` `class` `Node {``        ``public` `int` `key;``        ``public` `Node next;``    ``};` `    ``static` `Node newNode(``int` `key)``    ``{``        ``Node temp = ``new` `Node();``        ``temp.key = key;``        ``temp.next = ``null``;``        ``return` `temp;``    ``}` `    ``// A utility function to print a linked list``    ``static` `void` `printList(Node head)``    ``{``        ``while` `(head != ``null``) {``            ``Console.Write(head.key + ``" "``);``            ``head = head.next;``        ``}``        ``Console.WriteLine();``    ``}` `    ``// Function to detect first node of loop``    ``// in a linked list that may contain loop``    ``static` `Boolean detectLoop(Node head)``    ``{` `        ``// Create a temporary node``        ``Node temp = ``new` `Node();``        ``while` `(head != ``null``) {` `            ``// This condition is for the case``            ``// when there is no loop``            ``if` `(head.next == ``null``) {``                ``return` `false``;``            ``}` `            ``// Check if next is already``            ``// pointing to temp``            ``if` `(head.next == temp) {``                ``return` `true``;``            ``}` `            ``// Store the pointer to the next node``            ``// in order to get to it in the next step``            ``Node next = head.next;` `            ``// Make next point to temp``            ``head.next = temp;` `            ``// Get to the next node in the list``            ``head = nex;``        ``}` `        ``return` `false``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``Node head = newNode(1);``        ``head.next = newNode(2);``        ``head.next.next = newNode(3);``        ``head.next.next.next = newNode(4);``        ``head.next.next.next.next = newNode(5);` `        ``// Create a loop for testing(5 is pointing to 3)``        ``head.next.next.next.next.next = head.next.next;` `        ``Boolean found = detectLoop(head);``        ``if` `(found) {``            ``Console.WriteLine(``"Loop Found"``);``        ``}``        ``else` `{``            ``Console.WriteLine(``"No Loop"``);``        ``}``    ``}``}` `// This code is contributed by Princi Singh`

## Javascript

 ``

Output

`Loop Found`

Time complexity: O(N). Only one traversal of the loop is needed.
Auxiliary Space: O(1)

## Detect loop in a linked list by Storing length:

The idea is to store the length of the list from first node and last node, increment last node till reaches NULL or number of nodes in last is greater the current between first and last nodes.

Follow the steps below to solve the problem:

• In this method, two pointers are created, first (always points to head) and last.
• Each time the last pointer moves calculate no of nodes between the first and last.
• check whether the current no of nodes > previous no of nodes
• If yes we proceed by moving the last pointer
• Else it means we’ve reached the end of the loop, so return output accordingly.

Below is the implementation of the above approach:

## C++

 `// C++ program to return first node of loop``#include ``using` `namespace` `std;` `struct` `Node {``    ``int` `key;``    ``struct` `Node* next;``};` `Node* newNode(``int` `key)``{``    ``Node* temp = ``new` `Node;``    ``temp->key = key;``    ``temp->next = NULL;``    ``return` `temp;``}` `// A utility function to print a linked list``void` `printList(Node* head)``{``    ``while` `(head != NULL) {``        ``cout << head->key << ``" "``;``        ``head = head->next;``    ``}``    ``cout << endl;``}` `/*returns distance between first and last node every time`` ``* last node moves forwards*/``int` `distance(Node* first, Node* last)``{``    ``/*counts no of nodes between first and last*/``    ``int` `counter = 0;` `    ``Node* curr;``    ``curr = first;` `    ``while` `(curr != last) {``        ``counter += 1;``        ``curr = curr->next;``    ``}` `    ``return` `counter + 1;``}` `// Function to detect first node of loop``// in a linked list that may contain loop``bool` `detectLoop(Node* head)``{` `    ``// Create a temporary node``    ``Node* temp = ``new` `Node;` `    ``Node *first, *last;` `    ``/*first always points to head*/``    ``first = head;``    ``/*last pointer initially points to head*/``    ``last = head;` `    ``/*current_length stores no of nodes between current``     ``* position of first and last*/``    ``int` `current_length = 0;` `    ``/*prev_length stores no of nodes between previous``     ``* position of first and last*/``    ``int` `prev_length = -1;` `    ``while` `(current_length > prev_length && last != NULL) {``        ``// set prev_length to current length then update the``        ``// current length``        ``prev_length = current_length;``        ``// distance is calculated``        ``current_length = distance(first, last);``        ``// last node points the next node``        ``last = last->next;``    ``}` `    ``if` `(last == NULL) {``        ``return` `false``;``    ``}``    ``else` `{``        ``return` `true``;``    ``}``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(1);``    ``head->next = newNode(2);``    ``head->next->next = newNode(3);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(5);` `    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head->next->next->next->next->next = head->next->next;` `    ``bool` `found = detectLoop(head);``    ``if` `(found)``        ``cout << ``"Loop Found"``;``    ``else``        ``cout << ``"No Loop Found"``;` `    ``return` `0;``}`

## C

 `// C program to return first node of loop``#include ``#include ``#include ` `typedef` `struct` `Node {``    ``int` `key;``    ``struct` `Node* next;``} Node;` `Node* newNode(``int` `key)``{``    ``Node* temp = (Node*)``malloc``(``sizeof``(Node));``    ``temp->key = key;``    ``temp->next = NULL;``    ``return` `temp;``}` `// A utility function to print a linked list``void` `printList(Node* head)``{``    ``while` `(head != NULL) {``        ``printf``(``"%d "``, head->key);``        ``head = head->next;``    ``}``    ``printf``(``"\n"``);``}` `/*returns distance between first and last node every time`` ``* last node moves forwards*/``int` `distance(Node* first, Node* last)``{``    ``/*counts no of nodes between first and last*/``    ``int` `counter = 0;` `    ``Node* curr;``    ``curr = first;` `    ``while` `(curr != last) {``        ``counter += 1;``        ``curr = curr->next;``    ``}` `    ``return` `counter + 1;``}` `// Function to detect first node of loop``// in a linked list that may contain loop``bool` `detectLoop(Node* head)``{``    ``// Create a temporary node``    ``Node* temp = (Node *)``malloc``(``sizeof``(Node));``    ``Node *first, *last;` `    ``/*first always points to head*/``    ``first = head;``    ``/*last pointer initially points to head*/``    ``last = head;` `    ``/*current_length stores no of nodes between current``     ``* position of first and last*/``    ``int` `current_length = 0;` `    ``/*prev_length stores no of nodes between previous``     ``* position of first and last*/``    ``int` `prev_length = -1;` `    ``while` `(current_length > prev_length && last != NULL) {``        ``// set prev_length to current length then update the``        ``// current length``        ``prev_length = current_length;``        ``// distance is calculated``        ``current_length = distance(first, last);``        ``// last node points the next node``        ``last = last->next;``    ``}` `    ``if` `(last == NULL)``        ``return` `false``;``    ``else``        ``return` `true``;``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(1);``    ``head->next = newNode(2);``    ``head->next->next = newNode(3);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(5);` `    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head->next->next->next->next->next = head->next->next;` `    ``bool` `found = detectLoop(head);``    ``if` `(found)``        ``printf``(``"Loop Found"``);``    ``else``        ``printf``(``"No Loop Found"``);` `    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## Java

 `// Java program to return first node of loop``import` `java.util.*;``class` `GFG``{``static` `class` `Node``{``    ``int` `key;``    ``Node next;``};` `static` `Node newNode(``int` `key)``{``    ``Node temp = ``new` `Node();``    ``temp.key = key;``    ``temp.next = ``null``;``    ``return` `temp;``}` `// A utility function to print a linked list``static` `void` `printList(Node head)``{``    ``while` `(head != ``null``)``    ``{``        ``System.out.print(head.key + ``" "``);``        ``head = head.next;``    ``}``    ``System.out.println();``}` `/*returns distance between first and last node every time`` ``* last node moves forwards*/``static` `int` `distance(Node first, Node last)``{``  ` `    ``/*counts no of nodes between first and last*/``    ``int` `counter = ``0``;``    ``Node curr;``    ``curr = first;``    ``while` `(curr != last)``    ``{``        ``counter += ``1``;``        ``curr = curr.next;``    ``}``    ``return` `counter + ``1``;``}` `// Function to detect first node of loop``// in a linked list that may contain loop``static` `boolean` `detectLoop(Node head)``{` `    ``// Create a temporary node``    ``Node temp = ``new` `Node();``    ``Node first, last;` `    ``/*first always points to head*/``    ``first = head;``  ` `    ``/*last pointer initially points to head*/``    ``last = head;` `    ``/*current_length stores no of nodes between current``     ``* position of first and last*/``    ``int` `current_length = ``0``;` `    ``/*current_length stores no of nodes between previous``     ``* position of first and last*/``    ``int` `prev_length = -``1``;``    ``while` `(current_length > prev_length && last != ``null``)``    ``{``      ` `        ``// set prev_length to current length then update the``        ``// current length``          ``prev_length = current_length;``      ` `        ``// distance is calculated``        ``current_length = distance(first, last);``      ` `        ``// last node points the next node``        ``last = last.next;``    ``}``    ` `    ``if` `(last == ``null``)``    ``{``        ``return` `false``;``    ``}``    ``else``    ``{``        ``return` `true``;``    ``}``}` `/* Driver program to test above function*/``public` `static` `void` `main(String[] args)``{``    ``Node head = newNode(``1``);``    ``head.next = newNode(``2``);``    ``head.next.next = newNode(``3``);``    ``head.next.next.next = newNode(``4``);``    ``head.next.next.next.next = newNode(``5``);` `    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head.next.next.next.next.next = head.next.next;``    ``boolean` `found = detectLoop(head);``    ``if` `(found)``        ``System.out.print(``"Loop Found"``);``    ``else``        ``System.out.print(``"No Loop Found"``);``}``}` `// This code is contributed by gauravrajput1`

## Python3

 `# Python program to return first node of loop``class` `newNode:``    ``def` `__init__(``self``, key):``        ``self``.key ``=` `key``        ``self``.left ``=` `None``        ``self``.right ``=` `None``        `  `# A utility function to print a linked list``def` `printList(head):` `    ``while` `(head !``=` `None``) :``        ``print``(head.key, end``=``" "``)``        ``head ``=` `head.``next``;``    ` `    ``print``()`  `# returns distance between first and last node every time``# last node moves forwards``def` `distance(first, last):` `    ``# counts no of nodes between first and last``    ``counter ``=` `0` `    ``curr ``=` `first` `    ``while` `(curr !``=` `last):``        ``counter ``=` `counter ``+` `1``        ``curr ``=` `curr.``next``    `  `    ``return` `counter ``+` `1` `  ` `# Function to detect first node of loop``# in a linked list that may contain loop``def` `detectLoop(head):` `    ``# Create a temporary node``    ``temp ``=` `""` `    ``# first always points to head``    ``first ``=` `head;``    ``# last pointer initially points to head``    ``last ``=` `head;` `    ``# current_length stores no of nodes between current``    ``# position of first and last``    ``current_length ``=` `0` `    ``#current_length stores no of nodes between previous``    ``# position of first and last*/``    ``prev_length ``=` `-``1` `    ``while` `(current_length > prev_length ``and` `last !``=` `None``) :``        ``# set prev_length to current length then update the``        ``# current length``        ``prev_length ``=` `current_length``        ``# distance is calculated``        ``current_length ``=` `distance(first, last)``        ``# last node points the next node``        ``last ``=` `last.``next``;``    ` `    ` `    ``if` `(last ``=``=` `None``) :``        ``return` `False``    ` `    ``else` `:``        ``return` `True`` `  `# Driver program to test above function` `head ``=` `newNode(``1``);``head.``next` `=` `newNode(``2``);``head.``next``.``next` `=` `newNode(``3``);``head.``next``.``next``.``next` `=` `newNode(``4``);``head.``next``.``next``.``next``.``next` `=` `newNode(``5``);` `# Create a loop for testing(5 is pointing to 3)``head.``next``.``next``.``next``.``next``.``next` `=` `head.``next``.``next``;` `found ``=` `detectLoop(head)``if` `(found) :``    ``print``(``"Loop Found"``)``else` `:``    ``print``(``"No Loop Found"``)` `# This code is contributed by ihritik`

## C#

 `// C# program to return first node of loop``using` `System;` `public` `class` `GFG``{``  ``public``    ``class` `Node``    ``{``      ``public``        ``int` `key;``      ``public``        ``Node next;``    ``};` `  ``static` `Node newNode(``int` `key)``  ``{``    ``Node temp = ``new` `Node();``    ``temp.key = key;``    ``temp.next = ``null``;``    ``return` `temp;``  ``}` `  ``// A utility function to print a linked list``  ``static` `void` `printList(Node head)``  ``{``    ``while` `(head != ``null``)``    ``{``      ``Console.Write(head.key + ``" "``);``      ``head = head.next;``    ``}``    ``Console.WriteLine();``  ``}` `  ``/*returns distance between first and last node every time`` ``* last node moves forwards*/``  ``static` `int` `distance(Node first, Node last)``  ``{` `    ``/*counts no of nodes between first and last*/``    ``int` `counter = 0;``    ``Node curr;``    ``curr = first;``    ``while` `(curr != last)``    ``{``      ``counter += 1;``      ``curr = curr.next;``    ``}``    ``return` `counter + 1;``  ``}` `  ``// Function to detect first node of loop``  ``// in a linked list that may contain loop``  ``static` `bool` `detectLoop(Node head)``  ``{` `    ``// Create a temporary node``    ``Node temp = ``new` `Node();``    ``Node first, last;` `    ``/*first always points to head*/``    ``first = head;` `    ``/*last pointer initially points to head*/``    ``last = head;` `    ``/*current_length stores no of nodes between current``     ``* position of first and last*/``    ``int` `current_length = 0;` `    ``/*current_length stores no of nodes between previous``     ``* position of first and last*/``    ``int` `prev_length = -1;``    ``while` `(current_length > prev_length && last != ``null``)``    ``{` `      ``// set prev_length to current length then update the``      ``// current length``      ``prev_length = current_length;` `      ``// distance is calculated``      ``current_length = distance(first, last);` `      ``// last node points the next node``      ``last = last.next;``    ``}` `    ``if` `(last == ``null``)``    ``{``      ``return` `false``;``    ``}``    ``else``    ``{``      ``return` `true``;``    ``}``  ``}` `  ``/* Driver program to test above function*/``  ``public` `static` `void` `Main(String[] args)``  ``{``    ``Node head = newNode(1);``    ``head.next = newNode(2);``    ``head.next.next = newNode(3);``    ``head.next.next.next = newNode(4);``    ``head.next.next.next.next = newNode(5);` `    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head.next.next.next.next.next = head.next.next;``    ``bool` `found = detectLoop(head);``    ``if` `(found)``      ``Console.Write(``"Loop Found"``);``    ``else``      ``Console.Write(``"No Loop Found"``);``  ``}``}` `// This code is contributed by gauravrajput1`

## Javascript

 ``

Output

`Loop Found`

Time complexity: O(N2), For every node calculate the length of that node from the head by traversing.
Auxiliary Space: O(1)

## Detect loop in a linked list by Modifying Value:

The idea is to modify the value of the visited node and check if current nodes value is equal to that value or not.

Follow the steps below to solve the problem:

• Traverse the linked list and change the value(data) of that node to -1.
• Now, before modifying the value check whether the value of the node is already -1
• If it is -1 then return TRUE
• Otherwise, change the value of the node.
• Traverse the linked list till it reaches NULL.

Below is the implementation of the above approach:

## C++

 `// C++ program to return first node of loop``#include ``using` `namespace` `std;` `struct` `Node {``    ``int` `key;``    ``struct` `Node* next;``};` `Node* newNode(``int` `key)``{``    ``Node* temp = ``new` `Node;``    ``temp->key = key;``    ``temp->next = NULL;``    ``return` `temp;``}` `// Function to detect first node of loop``// in a linked list that may contain loop``bool` `detectLoop(Node* head)``{` `    ``// If the head is null we will return false``    ``if` `(!head)``        ``return` `0;``    ``else` `{` `        ``// Traversing the linked list``        ``// for detecting loop``        ``while` `(head) {``            ``// If loop found``            ``if` `(head->key == -1) {``                ``return` `true``;``            ``}` `            ``// Changing the data of visited node to any``            ``// value which is outside th given range here it``            ``// is supposed the given range is (1 <= Data on``            ``// Node <= 10^3)``            ``else` `{``                ``head->key = -1;``                ``head = head->next;``            ``}``        ``}``        ``// If loop not found return false``        ``return` `0;``    ``}``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(1);``    ``head->next = newNode(2);``    ``head->next->next = newNode(3);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(5);` `    ``/* Create a loop for testing(5 is pointing to 3) */``    ``head->next->next->next->next->next = head->next->next;` `    ``bool` `found = detectLoop(head);``    ``cout << (found ? ``"Loop Found"` `: ``"Not Found"``) << endl;``    ``return` `0;``}`

## Java

 `// Java program to return first node of loop``import` `java.util.*;` `class` `LinkedList {` `    ``// Head of list``    ``static` `Node head;` `    ``// Linked list Node``    ``static` `class` `Node {``        ``int` `data;``        ``Node next;` `        ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Inserts a new Node at front of the list. */``    ``static` `public` `void` `push(``int` `new_data)``    ``{` `        ``/* 1 & 2: Allocate the Node &``                  ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``// Function to detect first node of loop``    ``// in a linked list that may contain loop``    ``static` `boolean` `detectLoop(Node h)``    ``{` `        ``// If the head is null we will return false``        ``if` `(head == ``null``)``            ``return` `false``;``        ``else` `{` `            ``// Traversing the linked list``            ``// for detecting loop``            ``while` `(head != ``null``) {` `                ``// If loop found``                ``if` `(head.data == -``1``) {``                    ``return` `true``;``                ``}` `                ``// Changing the data of visited node to any``                ``// value which is outside th given range``                ``// here it is supposed the given range is (1``                ``// <= Data on Node <= 10^3)``                ``else` `{``                    ``head.data = -``1``;``                    ``head = head.next;``                ``}``            ``}` `            ``// If loop not found return false``            ``return` `false``;``        ``}``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``LinkedList llist = ``new` `LinkedList();` `        ``llist.push(``1``);``        ``llist.push(``2``);``        ``llist.push(``3``);``        ``llist.push(``4``);``        ``llist.push(``5``);` `        ``/* Create a loop for testing */``        ``llist.head.next.next.next.next.next``            ``= llist.head.next.next;` `        ``if` `(detectLoop(llist.head))``            ``System.out.println(``"Loop Found"``);``        ``else``            ``System.out.println(``"Not Found"``);``    ``}``}` `// This code is contributed by RohitOberoi`

## Python3

 `# Python program to return first node of loop``class` `Node:``    ``def` `__init__(``self``, d):``        ``self``.data ``=` `d``        ``self``.``next` `=` `None`  `head ``=` `None`  `def` `push(new_data):``    ``global` `head``    ``new_node ``=` `Node(new_data)``    ``new_node.``next` `=` `head``    ``head ``=` `new_node`  `def` `detectLoop(h):``    ``global` `head` `    ``if` `(head ``=``=` `None``):``        ``return` `False``    ``else``:` `        ``while` `(head !``=` `None``):``            ``if` `(head.data ``=``=` `-``1``):``                ``return` `True``            ``else``:``                ``head.data ``=` `-``1``                ``head ``=` `head.``next` `        ``return` `False`  `push(``1``)``push(``2``)``push(``3``)``push(``4``)``push(``5``)` `head.``next``.``next``.``next``.``next``.``next` `=` `head.``next``.``next` `if` `(detectLoop(head)):``    ``print``(``"Loop Found"``)``else``:``    ``print``(``"Not Found"``)` `    ``# This code is contributed by patel2127.`

## C#

 `// C# program to return first node of loop` `using` `System;` `public` `class` `Node {``    ``public` `int` `data;``    ``public` `Node next;` `    ``public` `Node(``int` `d)``    ``{``        ``data = d;``        ``next = ``null``;``    ``}``}` `public` `class` `GFG {` `    ``// Head of list``    ``static` `Node head;` `    ``/* Inserts a new Node at front of the list. */``    ``static` `public` `void` `push(``int` `new_data)``    ``{` `        ``/* 1 & 2: Allocate the Node &``                  ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``// Function to detect first node of loop``    ``// in a linked list that may contain loop``    ``static` `bool` `detectLoop(Node h)``    ``{` `        ``// If the head is null we will return false``        ``if` `(head == ``null``)``            ``return` `false``;``        ``else` `{` `            ``// Traversing the linked list``            ``// for detecting loop``            ``while` `(head != ``null``) {` `                ``// If loop found``                ``if` `(head.data == -1) {``                    ``return` `true``;``                ``}` `                ``// Changing the data of visited node to any``                ``// value which is outside th given range``                ``// here it is supposed the given range is (1``                ``// <= Data on Node <= 10^3)``                ``else` `{``                    ``head.data = -1;``                    ``head = head.next;``                ``}``            ``}` `            ``// If loop not found return false``            ``return` `false``;``        ``}``    ``}` `    ``// Driver Code` `    ``static` `public` `void` `Main()``    ``{` `        ``push(1);``        ``push(2);``        ``push(3);``        ``push(4);``        ``push(5);` `        ``/* Create a loop for testing */``        ``head.next.next.next.next.next = head.next.next;` `        ``if` `(detectLoop(head))``            ``Console.WriteLine(``"Loop Found"``);``        ``else``            ``Console.WriteLine(``"Not Found"``);``    ``}``}`

## Javascript

 ``

Output

```Loop Found
```

Time Complexity: O(N)
Auxiliary Space: O(1)