1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16

/**
 * The fast can catch slow if it's a cycle linked list.
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null) return false;
        ListNode fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) return true;
        }
        return false;
    }
}