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],
]

 1 class Solution {
 2 public:
 3     vector<vector<int> > combine(int n, int k) {
 4         vector<vector<int> > res;
 5         vector<int> com;
 6         combineRE(n, k, 1, com, res);
 7         return res;
 8     }
 9     
10     void combineRE(int n, int k, int start, vector<int> &com, vector<vector<int> > &res) {
11         int m = com.size();
12         if(m == k) {
13             res.push_back(com);
14             return;
15         }
16         for(int i = start; i <= n-k+1+m; i++) {
17             com.push_back(i);
18             combineRE(n, k, i+1, com, res);
19             com.pop_back();
20         }
21     }
22 };