// @Title: 环形链表 II (Linked List Cycle II)
// @Author: qisiii
// @Date: 2024-09-04 17:10:23
// @Runtime: 0 ms
// @Memory: 43.6 MB
// @comment: 
// @flag: 
/**
 * 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 slow = head,fast = head;
        while(slow!=null&&fast!=null){
            slow=slow.next;
            fast=fast.next;
            if(fast!=null){
                fast=fast.next;
                if(slow==fast){
                    fast=head;
                    while(slow!=fast){
                        fast=fast.next;
                        slow=slow.next;
                    }
                    return fast;
                }
            }
        }
        return null;
    }
}