【leetcode】【141】Linked List Cycle

一、问题描述

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

Follow up:

Can you solve it without using extra space?

二、问题分析

典型的two points问题，通过设置快慢指针，如果有环，那么快指针总会追上慢指针；如果不存在环，那么快指针会先指向null。

三、Java AC代码

public boolean hasCycle(ListNode head) {
if (head == null || head.next == null) {
return false;
}
ListNode slow = head;
ListNode fast = head;
while (fast!=null && fast.next != null) {
fast = fast.next.next;
slow = slow.next;
if (fast == slow) {
return true;
}
}
return false;
}