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],
]

class Solution {
public:
    vector<vector<int> > result;
    vector<int> row;
    
    void dfs_combine(const int &n, const int &k, int first) {
        if (row.size() == k) {
            result.push_back(row);
            return;
        }
        int i;
        for (i=first;i<=n;i++) {
            row.push_back(i);
            dfs_combine(n, k, i+1);
            row.pop_back();
        }
    }
    vector<vector<int> > combine(int n, int k) {
        result.clear();
        row.clear();
        dfs_combine(n,k,1);
        return result;
    }
};