#141. linked list cycle

**<font color=red>难度:Easy</font>**

## 刷题内容

> 原题连接

*https://leetcode.com/problems/linked-list-cycle/
* 
> 内容描述

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

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

## 解题方案

> 思路
******- 时间复杂度: O(N)******- 空间复杂度: O(1)******

这题典型的快慢指针在链表中的运用，定义两个指针，每次循环，一个加1，另一个加2，若两指针相遇，则有环。


```cpp
/**
 * 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) {
        if(head == nullptr)
            return false;
        ListNode* l1 = head,*l2 = head ->next;
        while(true)
        {
            if(l2 == nullptr || l2 ->next == nullptr)
                return false;
            if(l1 == l2)
                return true;
            l1 = l1 ->next;
            l2 = l2 ->next ->next;
        }
    }
};
```