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) {
        // 快慢指针
        ListNode fastNode = head;
        ListNode slowNode = head;
	    // 如果没有环肯定是快指针先访问完成，&&号后面的判断是为了防止非空。
        while (fastNode != null && fastNode.next != null ) {
            fastNode = fastNode.next.next;
            slowNode = slowNode.next;

            if(fastNode == slowNode) {
                return true;
            }
        }
        return false;
    }
}