BM6 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) {
        //头结点不存在，也就是没有环
        if (head == null) {
            return false;
        }
        //如果头结点的next域存着头结点的地址，那就是与自身为环
        if (head.next == head) {
            return true;
        }
        
        //创建两个指针(快慢指针)，同时指向头结点
        ListNode fast = head;
        ListNode slow = head;
        
        //fast走两步会先一步走到链表尾部，slow走一步
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            //如果fast不为空且fast.next也不为空，fast一定会跟slow相遇
            if (fast == slow) {
                return true;
            }
        }

        /**
         * while循环走完，fast一定没有跟slow相遇
         */
        return false;
    }
}