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:
    std::vector<std::vector<int> > combine(int n, int k) {
		std::vector<std::vector<int>> result;
		std::vector<int> vec;
		dfs(n,k,1,0,result,vec);
		return result;
    }
private:
	void dfs(int n, int k, int bgn, int end,std::vector<std::vector<int>> &result,std::vector<int> &vec)
	{
		if(end == k)
		{
			result.push_back(vec);
		}
		for (int i = bgn; i <= n; i++)
		{
			vec.push_back(i);
			dfs(n,k,i+1,end+1,result,vec);
			vec.pop_back();
		}
	}
};

leetcode - Combinations