# 77. Combinations

## 1.問題

* 給予n與k, 回傳長度為k的組合

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LN9pl8471p42rUunP2n%2F-LNASe19Mjt-pwrsgF_G%2F%E8%9E%A2%E5%B9%95%E5%BF%AB%E7%85%A7%202018-09-24%20%E4%B8%8B%E5%8D%888.02.47.png?alt=media\&token=f44340a2-837d-47f8-bf89-98ecc2048a1b)

## 2.想法

* 提問:
* function header, parameter
* test input
* 說明想法 
  * DFS, 當累積數量為特定大小時返回, 但要注意要設定start
* 測試計算複雜度

## 3.程式碼

```
class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> record;
        combination(res, record, n, k, 0); 
        return res;
    }
private:
    void combination(vector<vector<int>>& res, vector<int>& record, int n, int k, int start)  {
        if (record.size() == k) {
            res.push_back(record);
            return;
        }
        
        for (int i = start; i < n ; i++) {
            record.push_back(i + 1);
            combination(res, record, n, k, i + 1); 
            record.pop_back();
        }
    }
};
```