组合

class Solution {
public:
    vector<vector<int>> res;
    void combineAux(vector<int>& tmp, int n, int k)
    {
        for(int i=n;i>=k;--i)
        {
            tmp[k-1] = i;
            if(k-1 == 0)
            {
                res.emplace_back(tmp);
                continue;
            }
            combineAux(tmp, i-1, k-1);
        }
    }
    vector<vector<int>> combine(int n, int k) {
        vector<int> vec(k);
        combineAux(vec, n, k);
        return res;
    }
};