# 77.Combinations

**77.Combinations**

难度:Medium

> 给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。

示例:

```
输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]
```

可以采用递归的方法，n个数里找k个数，可以分两类，第一类有n，第二类没有。\
第一类可以看做是n-1个数里找k-1个数。\
第二类可以看做是n-1个数里找k个数。\
以此方式进行递归。\
注意判断特殊情况。

```
class Solution {

public:
    vector<vector<int>> combine(int n, int k) {
       // cout<<n<<"  "<<k<<endl;
        if(n<k) 
        {
            vector<vector<int>>res;
            return res;
        }
        if(n==k) {
            vector<int >tmp;
            for(int i=1;i<=n;i++)
                        tmp.push_back(i);
            vector<vector<int>>res;
            res.push_back(tmp);
            return res;
        }
        if(k==1)
        {
            vector<vector<int>> res;
            for(int i=1;i<=n;i++)
            {
                vector<int>tmp(1,i);
                res.push_back(tmp);
            }
            return res;
        }

        vector<vector<int>> res1= combine(n-1, k);
        vector<vector<int>>res2= combine(n-1, k-1);
        for(auto v: res2)
        {
            v.push_back(n);
            res1.push_back(v);
        }


        return res1;
    }
};
```

> 执行用时 :164 ms, 在所有 C++ 提交中击败了46.10%的用户\
> 内存消耗 :54.7 MB, 在所有 C++ 提交中击败了22.81%的用户


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