leetcode第10题（linked-list-cycle）

题目：

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

Follow up:

Can you solve it without using extra space?

思路：跟上面题一样，只是不用求相遇点，第一次相遇就可以判断有环。

代码：

/**
* Definition for singly-linked list.
* class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
public boolean hasCycle(ListNode head){
if(head == null || head.next == null){
return false;
}
ListNode fast = head;
ListNode slow = head;
while(fast!=null && fast.next != null){
fast = fast.next.next;
slow = slow.next;
if(slow==fast){
return true;
}
}
return false;
}
}