[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 };