/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    // floyd判圈法，快慢指针
    // 设置快慢两个指针都指向头结点
    // 让一个快指针一次走两步，慢指针一次走一步
    // 两个指针相遇后，将快指针移到起点，一次走一步，相遇的位置就是链表中环的起始位置
    public ListNode detectCycle(ListNode head) {
       if (head == null || head.next == null) {
            return null;
       }
       ListNode fast = head;
       ListNode slow = head;
       while (fast != null && fast.next != null) { // fast需要走两步，所以需要判断fast != null && fast.next != null
            fast = fast.next.next;
            slow = slow.next;
            if (slow == fast) {
                fast = head;
               while (fast != slow) {
                fast = fast.next;
                slow = slow.next;
               } 
               return slow;
            }
       }
        return null;
    }
}