leetcode77 组合 / combinations

题目描述：

    简单的dfs题，代码如下：

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