时间限制：1秒 空间限制：32768K

题目描述

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

我的代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        //快慢指针能相遇说明有环
        if((head==nullptr)||(head->next==nullptr)||
           (head->next->next==nullptr))
            return false;
        ListNode* fast=head->next->next;
        ListNode* slow=head->next;
        while((fast->next)&&(fast->next->next)){
            if(fast==slow)
                return true;
            fast=fast->next->next;
            slow=slow->next;
        }
        return false;
    }
};

运行时间：19ms
占用内存：1224k