原题链接：Leecode 77. 组合

class Solution {
public:
    vector<vector<int>> res;
    vector<int> tmp;
    void dfs(int n,int num,int k,int start)
    {
        if(num==k)
        {
            res.push_back(tmp);
            return ;
        }
        if(start+k-num-1>n) return ;//剪枝
        for(int i=start;i<=n;i++)
        {
            tmp.push_back(i);
            dfs(n,num+1,k,i+1);
            tmp.pop_back();
        }
    }
    vector<vector<int>> combine(int n, int k) {
        dfs(n,0,k,1);
        return res;
    }
};