makeCombinations created by Hexe - https://repl.it/@Hexe/makeCombinations

makeCombinations.cpp

#include <iostream>
#include <vector>

using namespace std;

/***********************************************/
/* @Task: combination
 * @Input: n: number range 1 to n; k is number takes
 * @Return: list of lists
 * @Example: leetcode 8.5
 */
/************************************************/

class Solution {
public:
    vector<vector<int>> combine(int nums, int takes) {
        vector<vector<int>> results;
        vector<int> list;
        // call the package with n staring at 1 and count = 0
        DFS(nums, takes, 1, 0, list, results);
        return results;
    }

private:
    static void DFS(int &nums, int& takes, int start, int count,
             vector<int>& list, vector<vector<int>>& results) {

        // Demonstration print outs
        // for(auto j:list) { cout << j << ' '; }
        // cout << endl;

        // Termination Condition(TC) push this list back is it reaches the takes
        if (count == takes) {
            results.push_back(list);
            // return here to avoid extending the branch
            return;
        }
        // for each ele in nums, should be head once.
        for (int i = start; i <= nums; ++i) {
            // choose it
            list.push_back(i);
            DFS(nums, takes, i+1, count+1, list, results);
            // not choose it
            // pop the ele out so you can fill another one at the same location.
            list.pop_back();
        }
    }

};

int main() {

    auto result = Solution().combine(4, 2);

    // cout << "*************************************" << endl;
    for(auto list: result) {
        for (auto ele: list) {
            cout << ele << ' ';
        }
        cout << endl;
    }

    return 0;
}