# Find length of loop in a Linked List using Map

Write a program that checks whether a given Linked List contains a loop and if a loop is present then returns the count of nodes in the loop. For example, a loop is present in the below-linked list and the length of the loop is 4. If the loop is not present, then the function should return 0.

Approach: In this post, we will use the concept of Map to store the addresses of nodes present in the linked list as a key and their position as the values.

Below is the step-by-step approach:

1. Traverse every node of the linked list and maintain the position starting with one. Increment the position after every node.
2. Check whether that node is present in the Map or not.
3. If the map does not contain the address of that node, insert it into the map along with its position.
4. If the map already contains the address of that node, return the difference between their positions.
5. If no such node has been found, return 0.

Below is the implementation of the above approach:

 `// C++ program to find length of loop` `// in a linked list using Map`   `#include ` `using` `namespace` `std;`   `// Linked List node` `struct` `Node {` `    ``int` `data;` `    ``struct` `Node* next;`   `    ``Node(``int` `num)` `    ``{` `        ``data = num;` `        ``next = NULL;` `    ``}` `};`   `// Function detects and counts loop` `// nodes in the list. If loop is not there,` `// then returns 0` `int` `countNodesinLoop(``struct` `Node* head)` `{` `    ``struct` `Node* p = head;` `    ``int` `pos = 0;`   `    ``// Maintain a map to store addresses` `    ``// of node and their position` `    ``unordered_map m;`   `    ``// Traverse through the linked list` `    ``while` `(p != NULL) {`   `        ``// If the node is not present in the map` `        ``if` `(m.find(p) == m.end()) {` `            ``m[p] = pos;` `            ``pos++;` `        ``}`   `        ``// if the node is present` `        ``else` `{`   `            ``// Return difference between` `            ``// position of the present node and` `            ``// position where that node occured before` `            ``return` `(pos - m[p]);` `        ``}` `        ``p = p->next;` `    ``}`   `    ``// Return 0 to indicate` `    ``// there is no loop` `    ``return` `0;` `}`   `// Driver code` `int` `main()` `{` `    ``// Create nodes of the linked list` `    ``struct` `Node* head = ``new` `Node(1);` `    ``head->next = ``new` `Node(2);` `    ``head->next->next = ``new` `Node(3);` `    ``head->next->next->next = ``new` `Node(4);` `    ``head->next->next->next->next = ``new` `Node(5);`   `    ``// Create a loop for testing the function` `    ``head->next->next->next->next->next = head->next;`   `    ``// Call the function for the above linked list` `    ``cout << countNodesinLoop(head) << endl;`   `    ``return` `0;` `}`

 `// Java program to find length of loop` `// in a linked list using Map` `import` `java.util.*;` `import` `java.io.*;`   `class` `GFG{` `      `  `static` `class` `Node ` `{ ` `    ``int` `data; ` `    ``Node next; `   `    ``// Constructor ` `    ``Node(``int` `num)` `    ``{ ` `        ``data = num; ` `        ``next = ``null``; ` `    ``} ` `} ` `  `  `// Function detects and counts loop` `// nodes in the list. If loop is not there,` `// then returns 0` `public` `static` `int` `countNodesinLoop(Node head)` `{` `    ``Node p = head;` `    ``int` `pos = ``0``;` `    `  `    ``// Maintain a map to store addresses` `    ``// of node and their position` `    ``HashMap m = ``new` `HashMap();`   `    ``// Traverse through the linked list` `    ``while` `(p != ``null``) ` `    ``{` `        `  `        ``// If the node is not present in the map` `        ``if` `(!m.containsKey(p)) ` `        ``{` `            ``m.put(p, pos);` `            ``pos++;` `        ``}`   `        ``// If the node is present` `        ``else` `        ``{` `            `  `            ``// Return difference between` `            ``// position of the present ` `            ``// node and position where ` `            ``// that node occured before` `            ``return` `(pos - m.get(p));` `        ``}` `        ``p = p.next;` `    ``}`   `    ``// Return 0 to indicate` `    ``// there is no loop` `    ``return` `0``;` `}    `   `// Driver code` `public` `static` `void` `main (String[] args) ` `{` `    `  `      ``// Create nodes of the linked list` `      ``Node head = ``new` `Node(``1``); ` `    ``head.next = ``new` `Node(``2``);` `    ``head.next.next = ``new` `Node(``3``);` `    ``head.next.next.next = ``new` `Node(``4``);` `    ``head.next.next.next.next = ``new` `Node(``5``);`   `    ``// Create a loop for testing the function` `    ``head.next.next.next.next.next = head.next;`   `    ``// Call the function for the above linked list` `    ``System.out.println(countNodesinLoop(head));` `}` `}`   `// This code is contributed by adityapande88`

Output
```4

```

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