Find length of loop in a Linked List using Map

Write a program that checks whether a given Linked List contains 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 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:



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// C++ program to length of loop
// in a linked list using Map
  
#include <bits/stdc++.h>
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<Node*, int> 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;
}

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Output:

4

Similar Article: Find the length of a loop in a Linked list using Floyd’s Cycle detection algorithm

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