Leetcode-141. Linked List Cycle

前言：为了后续的实习面试，开始疯狂刷题，非常欢迎志同道合的朋友一起交流。因为时间比较紧张，目前的规划是先过一遍，写出能想到的最优算法，第二遍再考虑最优或者较优的方法。如有错误欢迎指正。博主首发CSDN，mcf171专栏。

博客链接：mcf171的博客

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

Follow up:
Can you solve it without using extra space?

这个题简单一点就是用一个Set保存访问过的点。Your runtime beats 3.79% of java submissions.

public class Solution {    public boolean hasCycle(ListNode head) { Set<ListNode> set = new HashSet<ListNode>();boolean flag = false;while(head != null){if(set.contains(head)) {flag = true;break;}else{set.add(head); head = head.next;flag = false;}}return flag;    }}

不使用额外空间的话，论坛一个人太机智了。见代码Your runtime beats 9.64% of java submissions.

public class Solution {    public boolean hasCycle(ListNode head) {        boolean flag = false;ListNode search = head;while(search != null){if(search.next == head) return true;ListNode temp = search.next;search.next = head;search = temp;}return flag;    }}