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) {
        //设置快慢指针
        if(head == null) return false;
        ListNode p1 = head;
        ListNode p2 = head;
        while(p1 != null && p2 != null){
            //因为要前进两步，所以要先判断p2.next是否为空，否则会越界
            if(p2.next == null){
                return false;
            }
            p1 = p1.next;
            p2 = p2.next.next;
            if(p1 == p2){
                return true;
            }
        }
        return false;
    }
}