/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
          
      //先判断头结点是否为空  如果为空返回false
        if(head==null||head.next==null){
            return false;
        }
      //建立快慢指针
        ListNode slow = head;
        ListNode fast = head.next;
        //如果快慢指针不相等则一直循环
        while (slow!=fast){
          //判断快指针，快指针的下一个结点是否为空
            if(fast == null || fast.next == null){
                return false;
            }
          
            slow = slow.next;
            fast =  fast.next.next;
            
        }
        return true;
    }
}