# 【LeetCode】 141. Linked List Cycle

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

> To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

> 給一linked list，判斷裡面有沒有cycle。

> 我們用一個整數pos去代表linked list的尾巴指回linked list前面的哪個位置，如果pos為-1，代表裡面沒有cycle。

## Example:

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.


Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.


Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.


## Solution

* 你可能覺得去判斷`pos`就好，但是你當然拿不到這個變數。
* 去用兩個點指向頭，一個一次走一步，一個一次走兩步。如果兩個走到同一個點，代表`linked list`裡面有`cycle`。
* 需要小心指到`null`的時候不能再跑向下一個。

### Code

/**
 * 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)return false;
        
        ListNode* one=head->next;
        ListNode* two=head->next;
        
        
        if(!one)return false;
        two = two->next;
        if(!two)return false;
        
        
        for(;one&&two;one=one->next,two=two->next)
        {
            two=two->next;
            if(!two)return false;
            if(one==two)return true;
        }
        return false;
    }
};

###### tags: `LeetCode` `C++`