LeetCode 141. Linked List Cycle

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

Follow up:

Can you solve it without using extra space?

分析：可以将结点存入数组中，此方法需要额外的空间，也可以用双指针，快慢指针同时向前跑，如果跑到结尾，就没环，如果被套圈，证明有环。

代码：

/** * 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) {        // 链表和树的问题，注意处理特殊情况        if(head == null || head.next == null) return false;        ListNode slow = head;        ListNode fast = head.next;        while(slow != fast && slow.next != null && fast.next != null && fast.next.next != null){// 注意各种情况            slow = slow.next;            fast = fast.next.next;        }        if(slow == fast) return true;        return false;    }}