leetcode

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) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }
}

定义一个ListNode：

    public static class ListNode {
        int val;
        ListNode next = null;
        public ListNode(int val) {
            this.val = val;
        }
    }