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], ]

class Solution {

public:

    vector<vector<int> >res;

    int N;

    int K;

    vector<vector<int> > combine(int n, int k)

    {

        if(n == 0 || k > n)

            return res;

        K = k;

        N= n;

        vector<int> v;

        DFS(1, v, 0);

        return res;

    }

    void DFS(int pos, vector<int> &v, int cnt)

    {

        if(cnt == K)

        {

            res.push_back(v);

            return;

        }

        for(int i = pos; i <= N; i++)

        {

            v.push_back(i);

            DFS(i + 1, v, cnt + 1);

            v.pop_back();

        }

    }

};