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# 77. Combinations

## 1.問題

* 給予n與k, 回傳長度為k的組合

![](/files/-LNASe19Mjt-pwrsgF_G)

## 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();
        }
    }
};
```


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