Data Structures & Algorithms in Java – Linked Lists- Linked List Cycle

Problem:

Given a linked list and the head node of the linked list find if there is a loop / cycle in the linked list, that is if the tail node points back to any of the previous nodes.

For example:

The below linked list has a cycle:

source: leetcode.com

Output is true for the above linked list.

Try out the solution here:

https://leetcode.com/problems/linked-list-cycle/

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Solution:

Hint:

  • Have two pointers , a slow pointer moving one node at a time and a fast pointer moving two nodes at a time. If they meet each other there is a cycle.

ALGORITHM:

STEP 1: Initialize two pointers , slow and fast pointing to the head

STEP 2: While the pointers are not null , increment slow pointer by one node and fast pointer by two nodes.

STEP 2a: If the pointers meet return true

STEP 3: Return false

Code:

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        
        
        
        ListNode slow = head;
        
        ListNode fast = head;
        
  
        while(fast!=null &&  fast.next != null ){
            
                      
         slow = slow.next;
        
         fast = fast.next.next;

            
            
            if(slow == fast) return true;
            
            
          
            
        }
        
        return false;
        
    }
}

Time complexity is O(n) where n is the number of nodes.

There is no extra space required apart from storing the slow and fast pointers.

That’s it!

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