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) {
            return false;
        }

	  	// 使用快慢指针来判断是否有环
        ListNode fast = head;
        ListNode slow = head;
		
	  	// 这里是用fast作为while的判断，因为fast比较特别，一下要判断当前和下个节点
        while(fast!=null && fast.next!=null) {
            fast = fast.next.next;
            slow = slow.next;

            if(fast == slow) {
                return true;
            }
        }
        return false;
    }
}