Method combine

77. Combinations/Solution.cpp:10–15

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> v;
        vector<int> buf;
        this->recur(n, k, v, buf);
        return v;
    }
//recur
    void recur(int n, int k, vector<vector<int>> & v, vector<int> & buf) {
        if ( k == 0 ) {