[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！

class Solution {
public:
    void getNextComb(vector<vector<int> > &res, vector<int> v, int n, int k, int idx) {
        if (idx > n) {
            return;
        }
        v.push_back(idx);
        if (v.size() == k) {
            res.push_back(v);
        }
        
        getNextComb(res, v, n, k, idx + 1);
        v.pop_back();
        getNextComb(res, v, n, k, idx + 1);
    }
    
    vector<vector<int> > combine(int n, int k) {
        vector<vector<int> > res;
        vector<int> v;
        int idx = 1;
        getNextComb(res, v, n, k, idx);
        return res;
    }
};