时间复杂度o（n）

Given a linked list, determine if it has a cycle in it.

Can you solve it without using extra space?

• 设置两个指针，一个快指针，每次走两步，一个慢指针，每次走一步
• 如果块指针==慢指针，那么包含环
• 注意判断空值和长度为一的情况

/**
 * 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;
        }

        ListNode fast = head;
        ListNode slow = head;

        while(fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if(slow == fast) {
                return true;
            }
        }

        return false;
    }
}