Leetcode – Linked List Cycle

Category: Algorithms   December 12, 2012

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;
    }
}

Related posts:

  1. LeetCode – Palindrome Linked List (Java)
  2. LeetCode – Remove Nth Node From End of List (Java)
  3. LeetCode – Odd Even Linked List (Java)
  4. LeetCode – Plus One Linked List (Java)
Category >> Algorithms  
If you want someone to read your code, please put the code inside <pre><code> and </code></pre> tags. For example: 
<pre><code> 
String foo = "bar";
</code></pre>
  • গোলাম মোস্তফা রুমি

    Here is my code:

    class Solution {
    public:
    bool hasCycle(ListNode *head) {
    while(head != NULL){
    if(head->val == 0) return true;
    else{
    head->val = 0;
    }
    head = head->next;
    }
    return false;
    }
    };

  • Prakash Jha

    when will fast or fast->next become null??

  • Marina


    public class LinkedListCycle {

    public boolean check(LinkedList list) {
    Node fast = list.getHead();
    Node slow = list.getHead();

    while (fast != null && fast.getNext() != null && fast.getNext().getNext() != null) {
    if (slow == fast) {
    return true;
    }
    }
    return false;
    }
    }

  • 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.