题目：
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
给定一个链表，判断它是否有环
拓展：
你能不使用额外空间去解决这个问题吗？

思路：
快慢指针同时从头出发，能相遇则说明有环

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(fast == slow)
                return true;
        }
        return false;
    }
}