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) {
        if(head == null || head.next == null) {
            return false;
        }
	  // 双自增，只有当slow ！= fast，并且fast的next不为空死才表示不是环形
        ListNode slow = head.next;
        ListNode fast = slow.next;
        while(fast != null) {
            if(fast == slow) {
                return true;
            }
            if(slow.next == null || fast.next == null || fast.next.next == null) {
                return false;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        return false;
    }
}