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

Follow up:

Can you solve it without using extra space?

原题链接：https://oj.leetcode.com/problems/linked-list-cycle/

题目：给定一个链表。推断它是否有环。

继续：

你能不用额外的空间解决吗？

思路：使用两个指针fast,slow，fast每次向前走两步，slow每次向前走一步。假设二者相应的节点相等。即存在环。

	public boolean hasCycle(ListNode head) {

		ListNode fast = head,slow = head;

		if(head == null || head.next == null)

			return false;

		while(fast != null && fast.next != null){

			slow = slow.next;

			fast = fast.next.next;

			if(slow == fast)

				return true;

		}

		return false;

	}

	// Definition for singly-linked list.

	class ListNode {

		int val;

		ListNode next;

		ListNode(int x) {

			val = x;

			next = null;

		}

	}