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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Tayyab

Oh yes, the two B’s are the same node! That is what I was skipping.
Thank you so much! This bugged me for a long time.
Vani

Since it is cycle , next iternation, F jumps by skipping one element. So it skips, B and then goes to C. Remember that E connects back to B and next of E is B, next of B is C.
Tayyab

After F=E, how does it get to C again, if it’s skipping to the end of the list?
And even if it is starting form the beginning again, shouldn’t it again jump to A instead of C?
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.

Leetcode – Linked List Cycle

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