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],
]

Code:

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