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],
]

 

Runtime: 12ms

 1 class Solution {
 2 public:
 3     vector<vector<int>> combine(int n, int k) {
 4         vector<vector<int> > result;
 5         if(n < k || n <= 0) return result;
 6         
 7         vector<int> temp;
 8         helper(result, temp, n, k, 1);
 9         return result;
10     }
11     
12     void helper(vector<vector<int> >& result, vector<int>& temp, int n, int k, int depth){
13         if(temp.size() == k){
14             result.push_back(temp);
15             return;
16         }
17         
18         for(int i = depth; i <= n; i++){
19             temp.push_back(i);
20             helper(result, temp, n, k, i + 1);
21             temp.pop_back();
22         }
23     }
24 };