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Detect Cycle in a Linked List using Map
  • Last Updated : 26 Jan, 2021
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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:

C++




// 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;
}

Java




// Java program to detect loop in
// given linked list using map
import java.util.*;
 
class GFG{
 
// Structure for a node in Linked List
static class Node
{
    int data;
    Node next;
};
 
// Function to create Linked List
// Node
static 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
static HashMap<Node, Boolean> vis = new HashMap<>();
static boolean flag = false;
 
// Function to check cycle in Linked
// List
static void check(Node head)
{
     
    // If head is null return ;
    if (head == null)
    {
        flag = false;
        return;
    }
 
    // Mark the incoming Node as
    // visited if it is not visited yet
    if (vis.containsKey(head))
    {
        vis.put(head, true);
        check(head.next);
    }
 
    // If a visited Node is found
    // Update the flag value to 1
    // and return ;
    else
    {
        flag = true;
        return;
    }
}
 
// Driver Code
public static void main(String[] args)
{
     
    // 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)
        System.out.print("Loop detected.");
 
    // If flag is false, No Loop
    // detected
    else
        System.out.print("No Loop Found.");
         
    System.out.println();
}
}
 
// This code is contributed by Rajput-Ji

C#




// C# program to detect loop in
// given linked list using map
using System;
using System.Collections.Generic;
 
class GFG{
 
// Structure for a node in Linked List
public class Node
{
    public int data;
    public Node next;
};
 
// Function to create Linked List
// Node
static 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
static Dictionary<Node,
                  Boolean> vis = new Dictionary<Node,
                                                Boolean>();
static bool flag = false;
 
// Function to check cycle in Linked
// List
static void check(Node head)
{
     
    // If head is null return ;
    if (head == null)
    {
        flag = false;
        return;
    }
 
    // Mark the incoming Node as
    // visited if it is not visited yet
    if (vis.ContainsKey(head))
    {
        vis.Add(head, true);
        check(head.next);
    }
 
    // If a visited Node is found
    // Update the flag value to 1
    // and return ;
    else
    {
        flag = true;
        return;
    }
}
 
// Driver Code
public static void Main(String[] args)
{
     
    // 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)
        Console.Write("Loop detected.");
 
    // If flag is false, No Loop
    // detected
    else
        Console.Write("No Loop Found.");
         
    Console.WriteLine();
}
}
 
// This code is contributed by gauravrajput1
Output: 
Loop detected.

 

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

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