[Leetcode] 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],
]

DFS！

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