Detect Cycle in a Linked List using Map

Given a Linked List, check if the linked list has a loop or not.

There are various methods shown here: Detect Cycle in Linked List

Example

Input: 20->4->54->6->NULL
Output: No loop is detected.
Explanation:
While traversing the linked list we reached the end of linked list. Therefore no loop is present in linked list.

Input: 20->4->5->10->20
Output: Loop detected.
Explanation:
While traversing the linked list we reached the node with value 10 is linked with the head node which depicts a loop in the linked list. Therefore loop is present in linked list.



Approach:

  1. Create a map which will store the visited node in linked list.
  2. Traverse the linked list and do the following:
    • Check whether the current node is present in the map or not.
    • If the current node is not present in the map then, insert the current node in the map.
    • If the Node is present in the map, the loop in a linked list is detected.
  3. If we reached to Null Node while traversing the linked list then, the given linked list has no loop present in it.

Below is the implementation of above approach:

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ program to detect loop in
// given linked list using map
#include <bits/stdc++.h>
using namespace std;
  
// Structure for a node in Linked List
struct Node {
    int data;
    Node* next;
};
  
// Function to create Linked List
// Node
Node* newNode(int d)
{
    Node* temp = new Node;
    temp->data = d;
    temp->next = NULL;
    return temp;
}
  
// Declaration of Map to keep
// mark of visited Node
map<Node*, bool> vis;
bool flag = 0;
  
// Function to check cycle in Linked
// List
void check(Node* head)
{
    // If head is NULL return ;
    if (head == NULL) {
        flag = 0;
        return;
    }
  
    // Mark the incoming Node as
    // visited if it is not visited yet
    if (!vis[head]) {
        vis[head] = true;
        check(head->next);
    }
  
    // If a visited Node is found
    // Update the flag value to 1
    // and return ;
    else {
        flag = 1;
        return;
    }
}
  
// Driver Code
int main()
{
    // Create a head Node
    Node* head = newNode(20);
  
    // Inserting Node in Linked List
    head->next = newNode(4);
    head->next->next = newNode(5);
    head->next->next->next = newNode(10);
  
    // Just to make a cycle
    head->next->next->next->next = head;
  
    // Function that detect cycle in
    // Linked List
    check(head);
  
    // If flag is true, loop is found
    if (flag)
        cout << "Loop detected.";
  
    // If flag is false, No Loop
    // detected
    else
        cout << "No Loop Found.";
    cout << endl;
  
    return 0;
}

chevron_right


Output:

Loop detected.

Time Complexity: O(N*log N)
Auxiliary Space: O(N)

Don’t stop now and take your learning to the next level. Learn all the important concepts of Data Structures and Algorithms with the help of the most trusted course: DSA Self Paced. Become industry ready at a student-friendly price.




My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.



Improved By : Code_Mech