题解 | 判断链表中是否有环

import java.util.*;
/**
 * 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) {
        ListNode slow =head;
        ListNode quick = head;
        while(quick!=null){
           slow=slow.next;
           if(quick.next==null){
              break;
           }
           quick=quick.next.next;
           if(slow==quick){
             return true;
           }
        }
        return false;
    }
}

一定要对快指针的next进行判空不然会报空指针