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# Detect and Remove Loop in a Linked List

Write a function detectAndRemoveLoop() that checks whether a given Linked List contains a loop and if the loop is present then remove the loop and return true. If the list doesn’t contain a loop then it returns false. The below diagram shows a linked list with a loop. detectAndRemoveLoop() must change the below list to 1->2->3->4->5->NULL.

We also recommend reading the following post as a prerequisite to the solution discussed here.
Write a C function to detect a loop in a linked list
Before trying to remove the loop, we must detect it. Techniques discussed in the above post can be used to detect loops. To remove the loop, all we need to do is to get a pointer to the last node of the loop. For example, a node with value 5 in the above diagram. Once we have a pointer to the last node, we can make the next of this node NULL and the loop is gone.
We can easily use Hashing or Visited node techniques (discussed in the above-mentioned post) to get the pointer to the last node. The idea is simple: the very first node whose next is already visited (or hashed) is the last node.
We can also use the Floyd Cycle Detection algorithm to detect and remove the loop. In Floyd’s algo, the slow and fast pointers meet at a loop node. We can use this loop node to remove the cycle. There are following two different ways of removing the loop when Floyd’s algorithm is used for loop detection.

Method 1 (Check one by one) We know that Floyd’s Cycle detection algorithm terminates when fast and slow pointers meet at a common point. We also know that this common point is one of the loop nodes (2 or 3 or 4 or 5 in the above diagram). Store the address of this in a pointer variable say ptr2. After that start from the head of the Linked List and check for nodes one by one if they are reachable from ptr2. Whenever we find a node that is reachable, we know that this node is the starting node of the loop in the Linked List and we can get the pointer to the previous of this node.

Output:

```Linked List after removing loop
50 20 15 4 10 ```

Method 2 (Better Solution)

1. This method is also dependent on Floyd’s Cycle detection algorithm.
2. Detect Loop using Floyd’s Cycle detection algorithm and get the pointer to a loop node.
3. Count the number of nodes in the loop. Let the count be k.
4. Fix one pointer to the head and another to a kth node from the head.
5. Move both pointers at the same pace, they will meet at the loop starting node.
6. Get a pointer to the last node of the loop and make the next of it NULL.

Thanks to WgpShashank for suggesting this method.
Below image is a dry run of the ‘remove loop’ function in the code :

## C++

 `#include ``using` `namespace` `std;` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `/* Function to remove loop. */``void` `removeLoop(``struct` `Node*, ``struct` `Node*);` `/* This function detects and removes loop in the list``  ``If loop was there in the list then it returns 1,``  ``otherwise returns 0 */``int` `detectAndRemoveLoop(``struct` `Node* list)``{``    ``struct` `Node *slow_p = list, *fast_p = list;` `    ``// Iterate and find if loop exists or not``    ``while` `(slow_p && fast_p && fast_p->next) {``        ``slow_p = slow_p->next;``        ``fast_p = fast_p->next->next;` `        ``/* If slow_p and fast_p meet at some point then there``           ``is a loop */``        ``if` `(slow_p == fast_p) {``            ``removeLoop(slow_p, list);` `            ``/* Return 1 to indicate that loop is found */``            ``return` `1;``        ``}``    ``}` `    ``/* Return 0 to indicate that there is no loop*/``    ``return` `0;``}` `/* Function to remove loop.`` ``loop_node --> Pointer to one of the loop nodes`` ``head -->  Pointer to the start node of the linked list */``void` `removeLoop(``struct` `Node* loop_node, ``struct` `Node* head)``{``    ``struct` `Node* ptr1 = loop_node;``    ``struct` `Node* ptr2 = loop_node;` `    ``// Count the number of nodes in loop``    ``unsigned ``int` `k = 1, i;``    ``while` `(ptr1->next != ptr2) {``        ``ptr1 = ptr1->next;``        ``k++;``    ``}` `    ``// Fix one pointer to head``    ``ptr1 = head;` `    ``// And the other pointer to k nodes after head``    ``ptr2 = head;``    ``for` `(i = 0; i < k; i++)``        ``ptr2 = ptr2->next;` `    ``/*  Move both pointers at the same pace,``      ``they will meet at loop starting node */``    ``while` `(ptr2 != ptr1) {``        ``ptr1 = ptr1->next;``        ``ptr2 = ptr2->next;``    ``}` `    ``// Get pointer to the last node``    ``while` `(ptr2->next != ptr1)``        ``ptr2 = ptr2->next;` `    ``/* Set the next node of the loop ending node``      ``to fix the loop */``    ``ptr2->next = NULL;``}` `/* Function to print linked list */``void` `printList(``struct` `Node* node)``{``    ``// Print the list after loop removal``    ``while` `(node != NULL) {``        ``cout << node->data << ``" "``;``        ``node = node->next;``    ``}``}` `struct` `Node* newNode(``int` `key)``{``    ``struct` `Node* temp = ``new` `Node();``    ``temp->data = key;``    ``temp->next = NULL;``    ``return` `temp;``}` `// Driver Code``int` `main()``{``    ``struct` `Node* head = newNode(50);``    ``head->next = newNode(20);``    ``head->next->next = newNode(15);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next->next = head->next->next;` `    ``detectAndRemoveLoop(head);` `    ``cout << ``"Linked List after removing loop \n"``;``    ``printList(head);``    ``return` `0;``}` `// This code has been contributed by Striver`

## Java

 `// Java program to detect and remove loop in linked list` `class` `LinkedList {` `    ``static` `Node head;` `    ``static` `class` `Node {` `        ``int` `data;``        ``Node next;` `        ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``// Function that detects loop in the list``    ``int` `detectAndRemoveLoop(Node node)``    ``{``        ``Node slow = node, fast = node;``        ``while` `(slow != ``null` `&& fast != ``null``               ``&& fast.next != ``null``) {``            ``slow = slow.next;``            ``fast = fast.next.next;` `            ``// If slow and fast meet at same point then loop``            ``// is present``            ``if` `(slow == fast) {``                ``removeLoop(slow, node);``                ``return` `1``;``            ``}``        ``}``        ``return` `0``;``    ``}` `    ``// Function to remove loop``    ``void` `removeLoop(Node loop, Node head)``    ``{``        ``Node ptr1 = loop;``        ``Node ptr2 = loop;` `        ``// Count the number of nodes in loop``        ``int` `k = ``1``, i;``        ``Node prevNode = ptr1;``        ``while` `(ptr1.next != ptr2) {``            ``// keeping track beforeing moving next``            ``prevNode = ptr1;``            ``ptr1 = ptr1.next;``            ``k++;``        ``}``        ``prevNode.next = ``null``;``    ``}` `    ``// Function to print the linked list``    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``System.out.print(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Driver program to test above functions``    ``public` `static` `void` `main(String[] args)``    ``{``        ``LinkedList list = ``new` `LinkedList();``        ``list.head = ``new` `Node(``50``);``        ``list.head.next = ``new` `Node(``20``);``        ``list.head.next.next = ``new` `Node(``15``);``        ``list.head.next.next.next = ``new` `Node(``4``);``        ``list.head.next.next.next.next = ``new` `Node(``10``);` `        ``// Creating a loop for testing``        ``head.next.next.next.next.next = head.next.next;``        ``list.detectAndRemoveLoop(head);``        ``System.out.println(``            ``"Linked List after removing loop : "``);``        ``list.printList(head);``    ``}``}` `// This code has been contributed by Mayank Jaiswal`

## Python3

 `# Python program to detect and remove loop in 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` `    ``def` `detectAndRemoveLoop(``self``):``        ``slow_p ``=` `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 and fast_p meet at some point then``            ``# there is a loop``            ``if` `slow_p ``=``=` `fast_p:``                ``self``.removeLoop(slow_p)``        ` `                ``# Return 1 to indicate that loop is found``                ``return` `1``        ` `        ``# Return 0 to indicate that there is no loop``        ``return` `0` `    ``# Function to remove loop``    ``# loop_node --> pointer to one of the loop nodes``    ``# head --> Pointer to the start node of the linked list``    ``def` `removeLoop(``self``, loop_node):``        ``ptr1 ``=` `loop_node``        ``ptr2 ``=` `loop_node``        ` `        ``# Count the number of nodes in loop``        ``k ``=` `1``        ``while``(ptr1.``next` `!``=` `ptr2):``            ``ptr1 ``=` `ptr1.``next``            ``k ``+``=` `1` `        ``# Fix one pointer to head``        ``ptr1 ``=` `self``.head``        ` `        ``# And the other pointer to k nodes after head``        ``ptr2 ``=` `self``.head``        ``for` `i ``in` `range``(k):``            ``ptr2 ``=` `ptr2.``next` `        ``# Move both pointers at the same place``        ``# they will meet at loop starting node``        ``while``(ptr2 !``=` `ptr1):``            ``ptr1 ``=` `ptr1.``next``            ``ptr2 ``=` `ptr2.``next` `        ``# Get pointer to the last node``        ``while``(ptr2.``next` `!``=` `ptr1):``            ``ptr2 ``=` `ptr2.``next` `        ``# Set the next node of the loop ending node``        ``# to fix the loop``        ``ptr2.``next` `=` `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 the LinkedList``    ``def` `printList(``self``):``        ``temp ``=` `self``.head``        ``while``(temp):``            ``print``(temp.data, end ``=` `' '``)``            ``temp ``=` `temp.``next`  `# Driver program``llist ``=` `LinkedList()``llist.push(``10``)``llist.push(``4``)``llist.push(``15``)``llist.push(``20``)``llist.push(``50``)` `# Create a loop for testing``llist.head.``next``.``next``.``next``.``next``.``next` `=` `llist.head.``next``.``next` `llist.detectAndRemoveLoop()` `print``(``"Linked List after removing loop"``)``llist.printList()` `# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`

## C#

 `// A C# program to detect and remove loop in linked list``using` `System;``public` `class` `LinkedList {` `    ``Node head;` `    ``public` `class` `Node {` `        ``public` `int` `data;``        ``public` `Node next;` `        ``public` `Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``// Function that detects loop in the list``    ``int` `detectAndRemoveLoop(Node node)``    ``{``        ``Node slow = node, fast = node;``        ``while` `(slow != ``null` `&& fast != ``null` `&& fast.next != ``null``) {``            ``slow = slow.next;``            ``fast = fast.next.next;` `            ``// If slow and fast meet at same``            ``// point then loop is present``            ``if` `(slow == fast) {``                ``removeLoop(slow, node);``                ``return` `1;``            ``}``        ``}``        ``return` `0;``    ``}` `    ``// Function to remove loop``    ``void` `removeLoop(Node loop, Node head)``    ``{``        ``Node ptr1 = loop;``        ``Node ptr2 = loop;` `        ``// Count the number of nodes in loop``        ``int` `k = 1, i;``        ``while` `(ptr1.next != ptr2) {``            ``ptr1 = ptr1.next;``            ``k++;``        ``}` `        ``// Fix one pointer to head``        ``ptr1 = head;` `        ``// And the other pointer to k nodes after head``        ``ptr2 = head;``        ``for` `(i = 0; i < k; i++) {``            ``ptr2 = ptr2.next;``        ``}` `        ``/* Move both pointers at the same pace,``        ``they will meet at loop starting node */``        ``while` `(ptr2 != ptr1) {``            ``ptr1 = ptr1.next;``            ``ptr2 = ptr2.next;``        ``}` `        ``// Get pointer to the last node``        ``while` `(ptr2.next != ptr1) {``            ``ptr2 = ptr2.next;``        ``}` `        ``/* Set the next node of the loop ending node``        ``to fix the loop */``        ``ptr2.next = ``null``;``    ``}` `    ``// Function to print the linked list``    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``Console.Write(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Driver program to test above functions``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``LinkedList list = ``new` `LinkedList();``        ``list.head = ``new` `Node(50);``        ``list.head.next = ``new` `Node(20);``        ``list.head.next.next = ``new` `Node(15);``        ``list.head.next.next.next = ``new` `Node(4);``        ``list.head.next.next.next.next = ``new` `Node(10);` `        ``// Creating a loop for testing``        ``list.head.next.next.next.next.next = list.head.next.next;``        ``list.detectAndRemoveLoop(list.head);``        ``Console.WriteLine(``"Linked List after removing loop : "``);``        ``list.printList(list.head);``    ``}``}` `// This code contributed by Rajput-Ji`

## Javascript

 ``

Output

```Linked List after removing loop
50 20 15 4 10 ```

Time Complexity: O(N), Where N is the number of nodes in the tree
Auxiliary Space: O(1)

Method 3 (Optimized Method 2: Without Counting Nodes in Loop)
We do not need to count the number of nodes in Loop. After detecting the loop, if we start the slow pointer from the head and move both slow and fast pointers at the same speed until fast don’t meet, they would meet at the beginning of the loop.

How does this work?
Let slow and fast meet at some point after Floyd’s Cycle finding algorithm. The below diagram shows the situation when the cycle is found.

Situation when cycle is found

We can conclude below from the above diagram

```Distance traveled by fast pointer = 2 * (Distance traveled
by slow pointer)

(m + n*x + k) = 2*(m + n*y + k)

Note that before meeting the point shown above, fast
was moving at twice speed.

x -->  Number of complete cyclic rounds made by
fast pointer before they meet first time

y -->  Number of complete cyclic rounds made by
slow pointer before they meet first time```

From the above equation, we can conclude below

```    m + k = (x-2y)*n

Which means m+k is a multiple of n.
Thus we can write, m + k = i*n or m = i*n - k.
Hence, distance moved by slow pointer: m, is equal to distance moved by fast pointer:
i*n - k or (i-1)*n + n - k (cover the loop completely i-1 times and start from n-k).```

So if we start moving both pointers again at same speed such that one pointer (say slow) begins from head node of linked list and other pointer (say fast) begins from meeting point. When the slow pointer reaches the beginning of the loop (has made m steps), the fast pointer would have made also moved m steps as they are now moving at the same pace. Since m+k is a multiple of n and fast starts from k, they would meet at the beginning. Can they meet before also? No because slow pointer enters the cycle first time after m steps.

## C++

 `// C++ program to detect and remove 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 and remove loop in a linked list that``// may contain loop``void` `detectAndRemoveLoop(Node* head)``{``    ``// If list is empty or has only one node without loop``    ``if` `(head == NULL || head->next == NULL)``        ``return``;` `    ``Node *slow = head, *fast = head;` `    ``// Move slow and fast 1 and 2 steps ahead respectively.``    ``slow = slow->next;``    ``fast = fast->next->next;` `    ``// Search for loop using slow and fast pointers``    ``while` `(fast && fast->next) {``        ``if` `(slow == fast)``            ``break``;``        ``slow = slow->next;``        ``fast = fast->next->next;``    ``}` `    ``/* If loop exists */``    ``if` `(slow == fast) {``        ``slow = head;` `        ``// this check is needed when slow and fast both meet``        ``// at the head of the LL eg: 1->2->3->4->5 and then``        ``// 5->next = 1 i.e the head of the LL``        ``if` `(slow == fast)``            ``while` `(fast->next != slow)``                ``fast = fast->next;``        ``else` `{``            ``while` `(slow->next != fast->next) {``                ``slow = slow->next;``                ``fast = fast->next;``            ``}``        ``}` `        ``/* since fast->next is the looping point */``        ``fast->next = NULL; ``/* remove loop */``    ``}``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(50);``    ``head->next = head;``    ``head->next = newNode(20);``    ``head->next->next = newNode(15);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next->next = head;` `    ``detectAndRemoveLoop(head);` `    ``printf``(``"Linked List after removing loop \n"``);``    ``printList(head);` `    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## C

 `// C++ program to detect and remove loop``#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 and remove loop in a linked list that``// may contain loop``void` `detectAndRemoveLoop(Node* head)``{``    ``// If list is empty or has only one node without loop``    ``if` `(head == NULL || head->next == NULL)``        ``return``;` `    ``Node *slow = head, *fast = head;` `    ``// Move slow and fast 1 and 2 steps ahead respectively.``    ``slow = slow->next;``    ``fast = fast->next->next;` `    ``// Search for loop using slow and fast pointers``    ``while` `(fast && fast->next) {``        ``if` `(slow == fast)``            ``break``;``        ``slow = slow->next;``        ``fast = fast->next->next;``    ``}` `    ``/* If loop exists */``    ``if` `(slow == fast) {``        ``slow = head;` `        ``// this check is needed when slow and fast both meet``        ``// at the head of the LL eg: 1->2->3->4->5 and then``        ``// 5->next = 1 i.e the head of the LL``        ``if` `(slow == fast)``            ``while` `(fast->next != slow)``                ``fast = fast->next;``        ``else` `{``            ``while` `(slow->next != fast->next) {``                ``slow = slow->next;``                ``fast = fast->next;``            ``}``        ``}` `        ``/* since fast->next is the looping point */``        ``fast->next = NULL; ``/* remove loop */``    ``}``}` `/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(50);``    ``head->next = head;``    ``head->next = newNode(20);``    ``head->next->next = newNode(15);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next->next = head;` `    ``detectAndRemoveLoop(head);` `    ``printf``(``"Linked List after removing loop \n"``);``    ``printList(head);` `    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## Java

 `// Java program to detect``// and remove loop in linked list` `class` `LinkedList {` `    ``static` `Node head;` `    ``static` `class` `Node {` `        ``int` `data;``        ``Node next;` `        ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``// Function that detects loop in the list``    ``void` `detectAndRemoveLoop(Node node)``    ``{` `        ``// If list is empty or has only one node``        ``// without loop``        ``if` `(node == ``null` `|| node.next == ``null``)``            ``return``;` `        ``Node slow = node, fast = node;` `        ``// Move slow and fast 1 and 2 steps``        ``// ahead respectively.``        ``slow = slow.next;``        ``fast = fast.next.next;` `        ``// Search for loop using slow and fast pointers``        ``while` `(fast != ``null` `&& fast.next != ``null``) {``            ``if` `(slow == fast)``                ``break``;` `            ``slow = slow.next;``            ``fast = fast.next.next;``        ``}` `         ``/* If loop exists */``        ``if` `(slow == fast) {``            ``slow = node;``            ``if` `(slow != fast) {``                ``while` `(slow.next != fast.next) {``                    ``slow = slow.next;``                    ``fast = fast.next;``                ``}``                ``/* since fast->next is the looping point */``                ``fast.next = ``null``; ``/* remove loop */``            ``}``              ``/* This case is added if fast and slow pointer meet at first position. */``            ``else` `{``                ``while``(fast.next != slow) {``                    ``fast = fast.next;``                ``}``                ``fast.next = ``null``;``            ``}``        ``}``    ``}` `    ``// Function to print the linked list``    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``System.out.print(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``LinkedList list = ``new` `LinkedList();``        ``list.head = ``new` `Node(``50``);``        ``list.head.next = ``new` `Node(``20``);``        ``list.head.next.next = ``new` `Node(``15``);``        ``list.head.next.next.next = ``new` `Node(``4``);``        ``list.head.next.next.next.next = ``new` `Node(``10``);` `        ``// Creating a loop for testing``        ``head.next.next.next.next.next = head.next.next;``        ``list.detectAndRemoveLoop(head);``        ``System.out.println(``"Linked List after removing loop : "``);``        ``list.printList(head);``    ``}``}` `// This code has been contributed by Mayank Jaiswal`

## Python3

 `# Python program to detect and remove loop` `# 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` `    ``def` `detectAndRemoveLoop(``self``): ``      ``if` `self``.head ``is` `None``:``          ``return``      ``if` `self``.head.``next` `is` `None``:``          ``return``      ``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 and fast_p meet at some point then``          ``# there is a loop``          ``if` `slow_p ``=``=` `fast_p:``            ``slow_p ``=` `self``.head``              ``# Finding the beginning of the loop``            ``while` `(slow_p.``next` `!``=` `fast_p.``next``):``              ``slow_p ``=` `slow_p.``next``              ``fast_p ``=` `fast_p.``next` `                ``# Sinc fast.next is the looping point``            ``fast_p.``next` `=` `None`  `# Remove loop` `    ``# Utility function to print the LinkedList` `    ``def` `printList(``self``):``        ``temp ``=` `self``.head``        ``while``(temp):``            ``print``(temp.data, end ``=` `' '``)``            ``temp ``=` `temp.``next`  `# Driver program``llist ``=` `LinkedList()``llist.head ``=` `Node(``50``)``llist.head.``next` `=` `Node(``20``)``llist.head.``next``.``next` `=` `Node(``15``)``llist.head.``next``.``next``.``next` `=` `Node(``4``)``llist.head.``next``.``next``.``next``.``next` `=` `Node(``10``)` `# Create a loop for testing``llist.head.``next``.``next``.``next``.``next``.``next` `=` `llist.head.``next``.``next` `llist.detectAndRemoveLoop()` `print``(``"Linked List after removing loop"``)``llist.printList()` `# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`

## C#

 `// C# program to detect and remove loop in linked list``using` `System;` `public` `class` `LinkedList {` `    ``public` `Node head;` `    ``public` `class` `Node {` `        ``public` `int` `data;``        ``public` `Node next;` `        ``public` `Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``// Function that detects loop in the list``    ``void` `detectAndRemoveLoop(Node node)``    ``{` `        ``// If list is empty or has only one node``        ``// without loop``        ``if` `(node == ``null` `|| node.next == ``null``)``            ``return``;` `        ``Node slow = node, fast = node;` `        ``// Move slow and fast 1 and 2 steps``        ``// ahead respectively.``        ``slow = slow.next;``        ``fast = fast.next.next;` `        ``// Search for loop using slow and fast pointers``        ``while` `(fast != ``null` `&& fast.next != ``null``) {``            ``if` `(slow == fast)``                ``break``;` `            ``slow = slow.next;``            ``fast = fast.next.next;``        ``}` `        ``/* If loop exists */``        ``if` `(slow == fast) {``            ``slow = node;``            ``while` `(slow.next != fast.next) {``                ``slow = slow.next;``                ``fast = fast.next;``            ``}` `            ``/* since fast->next is the looping point */``            ``fast.next = ``null``; ``/* remove loop */``        ``}``    ``}` `    ``// Function to print the linked list``    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``Console.Write(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Driver program to test above functions``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``LinkedList list = ``new` `LinkedList();``        ``list.head = ``new` `Node(50);``        ``list.head.next = ``new` `Node(20);``        ``list.head.next.next = ``new` `Node(15);``        ``list.head.next.next.next = ``new` `Node(4);``        ``list.head.next.next.next.next = ``new` `Node(10);` `        ``// Creating a loop for testing``        ``list.head.next.next.next.next.next = list.head.next.next;``        ``list.detectAndRemoveLoop(list.head);``        ``Console.WriteLine(``"Linked List after removing loop : "``);``        ``list.printList(list.head);``    ``}``}` `// This code contributed by Rajput-Ji`

## Javascript

 ``

Output

```Linked List after removing loop
50 20 15 4 10 ```

Time Complexity: O(N), Where N is the number of nodes in the tree
Auxiliary Space: O(1)

We can hash the addresses of the linked list nodes in an unordered map and just check if the element already exists in the map. If it exists, we have reached a node that already exists by a cycle, hence we need to make the last node’s next pointer NULL.

## C++

 `// C++ program to detect and remove 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 and remove loop``// in a linked list that may contain loop``void` `hashAndRemove(Node* head)``{``    ``// hash map to hash addresses of the linked list nodes``    ``unordered_map node_map;``    ``// pointer to last node``    ``Node* last = NULL;``    ``while` `(head != NULL) {``        ``// if node not present in the map, insert it in the map``        ``if` `(node_map.find(head) == node_map.end()) {``            ``node_map[head]++;``            ``last = head;``            ``head = head->next;``        ``}``        ``// if present, it is a cycle, make the last node's next pointer NULL``        ``else` `{``            ``last->next = NULL;``            ``break``;``        ``}``    ``}``}``/* Driver program to test above function*/``int` `main()``{``    ``Node* head = newNode(50);``    ``head->next = head;``    ``head->next = newNode(20);``    ``head->next->next = newNode(15);``    ``head->next->next->next = newNode(4);``    ``head->next->next->next->next = newNode(10);` `    ``/* Create a loop for testing */``    ``head->next->next->next->next->next = head->next->next;` `    ``// printList(head);``    ``hashAndRemove(head);` `    ``printf``(``"Linked List after removing loop \n"``);``    ``printList(head);` `    ``return` `0;``}`

## Java

 `// Java program to detect  and remove 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;``    ``}` `    ``// Function to print the linked list``    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``System.out.print(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Returns true if the loop is removed from the linked``    ``// list else returns false.``    ``static` `boolean` `removeLoop(Node h)``    ``{``        ``HashSet s = ``new` `HashSet();``        ``Node prev = ``null``;``        ``while` `(h != ``null``) {``            ``// If we have already has this node``            ``// in hashmap it means there is a cycle and we``            ``// need to remove this cycle so set the next of``            ``// the previous pointer with null.` `            ``if` `(s.contains(h)) {``                ``prev.next = ``null``;``                ``return` `true``;``            ``}` `            ``// If we are seeing the node for``            ``// the first time, insert it in hash``            ``else` `{``                ``s.add(h);``                ``prev = 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` `(removeLoop(head)) {``            ``System.out.println(``"Linked List after removing loop"``);``            ``llist.printList(head);``        ``}``        ``else``            ``System.out.println(``"No Loop found"``);``    ``}``}` `// This code is contributed by Animesh Nag.`

## C#

 `// C# program to detect  and remove loop in a linked list` `using` `System;``using` `System.Collections.Generic;``public` `class` `LinkedList {` `    ``public` `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``;``        ``}``    ``}` ` `  `    ``// Function to print the linked list``    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``Console.Write(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Returns true if the loop is removed from the linked``    ``// list else returns false.``    ``bool` `removeLoop(Node h)``    ``{``        ``HashSet s = ``new` `HashSet();``        ``Node prev = ``null``;``        ``while` `(h != ``null``) {``            ``// If we have already has this node``            ``// in hashmap it means there is a cycle and we``            ``// need to remove this cycle so set the next of``            ``// the previous pointer with null.` `            ``if` `(s.Contains(h)) {``                ``prev.next = ``null``;``                ``return` `true``;``            ``}` `            ``// If we are seeing the node for``            ``// the first time, insert it in hash``            ``else` `{``                ``s.Add(h);``                ``prev = h;``                ``h = h.next;``            ``}``        ``}` `        ``return` `false``;``    ``}` `    ``/* Driver program to test above function */``    ``public` `static` `void` `Main()``    ``{``       ` `        ``LinkedList list = ``new` `LinkedList();``        ``list.head = ``new` `Node(50);``        ``list.head.next = ``new` `Node(20);``        ``list.head.next.next = ``new` `Node(15);``        ``list.head.next.next.next = ``new` `Node(4);``        ``list.head.next.next.next.next = ``new` `Node(10);` `        ``/*Create loop for testing */``        ``list.head.next.next.next.next.next = list.head.next.next;` `        ``if` `(list.removeLoop(list.head)) {``            ``Console.WriteLine(``"Linked List after removing loop"``);``            ``list.printList(list.head);``        ``}``        ``else``            ``Console.WriteLine(``"No Loop found"``);``    ``}``}` `// This code is contributed by ihritik`

## Javascript

 ``

## Python

 `class` `LinkedList:``    ``head ``=` `None``    ``# head of list``    ``# Linked list Node` `    ``class` `Node:``        ``data ``=` `0``        ``next` `=` `None` `        ``def` `__init__(``self``, d):``            ``self``.data ``=` `d``            ``self``.``next` `=` `None``    ``# Inserts a new Node at front of the list.` `    ``@staticmethod``    ``def` `push(new_data):``        ``# 1 & 2: Allocate the Node &``        ``#                Put in the data``        ``new_node ``=` `LinkedList.Node(new_data)``        ``# 3. Make next of new Node as head``        ``new_node.``next` `=` `LinkedList.head``        ``# 4. Move the head to point to new Node``        ``LinkedList.head ``=` `new_node``    ``# Function to print the linked list` `    ``def` `printList(``self``, node):``        ``while` `(node !``=` `None``):``            ``print``(node.data)``            ``node ``=` `node.``next``    ``# Returns true if the loop is removed from the linked``    ``# list else returns false.` `    ``@staticmethod``    ``def` `removeLoop(h):``        ``s ``=` `set``()``        ``prev ``=` `None``        ``while` `(h !``=` `None``):``            ``# If we have already has this node``            ``# in hashmap it means there is a cycle and we``            ``# need to remove this cycle so set the next of``            ``# the previous pointer with null.``            ``if` `(h ``in` `s):``                ``prev.``next` `=` `None``                ``return` `True``            ``else``:``                ``s.add(h)``                ``prev ``=` `h``                ``h ``=` `h.``next``        ``return` `False``    ``# Driver program to test above function` `    ``@staticmethod``    ``def` `main(args):``        ``llist ``=` `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` `(LinkedList.removeLoop(LinkedList.head)):``            ``print``(``"Linked List after removing loop"``)``            ``llist.printList(LinkedList.head)``        ``else``:``            ``print``(``"No Loop found"``)`  `if` `__name__ ``=``=` `"__main__"``:``    ``LinkedList.main([])` `# This code is contributed by mukulsomukesh`

Output

```Linked List after removing loop
50 20 15 4 10 ```

Time Complexity: O(N), Where N is the number of nodes in the tree.
Auxiliary Space: O(N), Where N is the number of nodes in the tree (due to hashing).

Please write comments if you find the above codes/algorithms incorrect, or find other ways to solve the same problem