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?

方法就是两个指针以不同的步长（一个fast一次两个，slow一次一个）遍历链表，如果相等（fast=slow），则链表中存在环。

实现代码如下：

public class Solution {    public boolean hasCycle(ListNode head) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        ListNode slow = head;        ListNode fast = head;        while(fast != null && fast.next != null)        {            slow = slow.next;            fast = fast.next.next;            if(slow == fast)                return true;        }        return false;    }}