印象题

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode fast=head,slow=head;
        while(true){
            if(fast==null||fast.next==null) return null;
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow)break;
        }
        fast=head;
        while(slow!=fast){
            slow=slow.next;
            fast=fast.next;
        }
        return fast;
    }
}