Leetcode99: Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

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