77 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],
]

Solution-recursive

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> result;
        vector<int> tmp;
        if ( n < k or k == 0 ) {
            result.push_back(tmp);
            return result;
        }
        vector<vector<int>> help1 = combine(n-1, k-1);
        vector<vector<int>> help2 = combine(n-1, k);
        for ( int i = 0; i <= help1.size()-1; i++ ) {
            tmp = help1[i];
            tmp.push_back(n);
            result.push_back(tmp);
        }
        for ( int i = 0; i <= help2.size()-1; i++ ) {
            tmp = help2[i];
            if ( tmp.size() != 0 ) result.push_back(help2[i]);
        }
        return result;
    }
};

Solution-recusive, V2

class Solution {
public:
    void help(int n, int i0, int k, vector<int> curr, vector<vector<int>>& res) {
        if ( n-i0+1 < k ) return;
        if ( k == 0 ) {res.push_back(curr); return;}
        if ( i0 == n+1 ) return;
        help(n, i0+1, k, curr, res);
        curr.push_back(i0);
        help(n, i0+1, k-1, curr, res);
        return;
    }
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> curr;
        help(n, 1, k, curr, res);
        return res;
    }
};