Leetcode – Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Analysis

If we have 2 pointers - fast and slow. It is guaranteed that the fast one will meet the slow one if there exists a circle.

The problem can be demonstrated in the following diagram:
linked-list-cycle

Java Solution

public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
 
        while(fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
 
            if(slow == fast)
                return true;
        }
 
        return false;
    }
}
Category >> Algorithms  
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  • tm

    If you compare two nodes using boolean operators will it return the correct value? Shouldn’t you implement your own equal method for nodes?

  • Carlos Gomez

    Does this make sense?

    A -> B -> C -> D -> E -> B

    iter 1
    S = A
    F = C

    iter 2
    S = B
    F = E

    iter 3
    S = C
    F = C

    Cycle detected, return true

  • Tayyab

    This confuses me, what if there is a sixth element in the list that is looping to the third element?
    A->B->C->D->E->B

    fast will never reach the second B and the loop will terminate without detecting the loop.
    This solution is on so many sites, why can’t people see it?
    Or may be I’m catching something wrong?

  • ryanlr

    Thanks! fixed.

  • ryanlr

    Yes, deleted.

  • dd

    The naive version is wrong?

  • shadowlou

    The first two if statements are already included in the third one.