# Detect loop in a linked list

Given a linked list, check if the linked list has loop or not. Below diagram shows a linked list with a loop. ## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Following are different ways of doing this
Solution 1: Hashing
Approach:
Traverse the list one by one and keep putting the node addresses in a Hash Table. At any point, if NULL is reached then return false and if next of current node points to any of the previously stored nodes in Hash then return true.

## 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 we 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

 `# 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, end ``=``" "``) ` `            ``temp ``=` `temp.``next` `  `  `  `  `    ``def` `detectLoop(``self``): ` `         ``s ``=` `set``() ` `         ``temp ``=` `self``.head ` `         ``while` `(temp): ` `         `  `             ``# If we have already has ` `             ``# this node in hashmap it ` `             ``# means their is a cycle ` `             ``# (Because you we 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 `

Output:

```Loop found
```

Complexity Analysis:

• 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 hashmap.

Solution 2: This problem can be solved without hashmap by modifying the linked list data-structure.
Approach: This solution requires modifications to the basic linked list data structure.

• 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. This solution works in O(n) but requires additional information with each node.
• A variation of this solution that doesn’t require modification to basic data structure can be implemented using a hash, just store the addresses of visited nodes in a hash and if you see an address that already exists in hash then there is a loop.

 `// 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 `

Output:

```Loop Found
```

Complexity Analysis:

• 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 hashmap.

Solution 3: Floyd’s Cycle-Finding Algorithm
Approach: This is the fastest method and has been described below:

• 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 linked list doesn’t have a loop.

Below image shows how the detectloop 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 ` `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(``"Loop not found"``); ` `    ``} ` ` `  `    ``/* 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`  ` `  `# 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` `"Found Loop"` `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 `

Output:

```Found Loop
```

Complexity Analysis:

• Time complexity: O(n).
Only one traversal of the loop is needed.
• Auxiliary Space:O(1).
There is no space required.

How does above algorithm work?
Please See : How does Floyd’s slow and fast pointers approach work?

Solution 4: Marking visited nodes without modifying the linked list data structure
In this method, 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 loop doesn’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* nex = head->next; ` ` `  `        ``// Make next point to temp ` `        ``head->next = temp; ` ` `  `        ``// Get to the next node in the list ` `        ``head = nex; ` `    ``} ` ` `  `    ``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; ` `} `

## Java

 `// Java program to return first node of loop ` `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 nex = 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) ` `            ``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 pra 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 ` `        ``nex ``=` `head.``next` ` `  `        ``# Make next poto temp ` `        ``head.``next` `=` `temp ` ` `  `        ``# Get to the next node in the list ` `        ``head ``=` `nex ` ` `  `    ``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 nex = 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 `

Output:

```Loop Found
```

Complexity Analysis:

• Time complexity: O(n).
Only one traversal of the loop is needed.
• Auxiliary Space: O(1).
There is no space required.