[leetcode]141. Linked List Cycle判断链表是否有环

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

Follow up:
Can you solve it without using extra space?

题意：

给定一个链表，判断是否循环

思路：

快慢指针

若有环，则快慢指针一定会在某个节点相遇(此处省略证明)

代码：

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