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;    }};