Combinations [LeetCode]

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],
]

  Summary: recursive functions

 class Solution {
 public:
     vector<vector<int> > combine(int n, int k) {
         vector<vector<int> > combinations;
         if(n ==  || k ==  || n < k)
             return combinations;
         if(k == ){
             for(int i = ; i <= n; i ++) {
                 vector<int> com(, i);
                 combinations.push_back(com);
             }
             return combinations;
         }

         if(k == n){
             vector<int> com;
             for(int i = ; i <= n; i ++) {
                 com.push_back(i);
             }
             combinations.push_back(com);
             return combinations;
         }

         if(k <= n / ){
             for(int i = ; i <= n - k; i ++){
                 if(i == n - k){
                     vector<int> com;
                     for(int j = n - k + ; j <= n; j ++)
                         com.push_back(j);
                     combinations.push_back(com);
                     break;
                 }

                 int pick_num = i + ;
                 vector<vector<int> > sub_com = combine(n - pick_num, k - );
                 for(auto item : sub_com) {
                     for(int j = ; j < item.size(); j ++){
                         item[j] += pick_num;
                     }
                     item.insert(item.begin(), pick_num);
                     combinations.push_back(item);
                 }
             }
             return combinations;
         }else{
             combinations = combine(n, n - k);
             vector<vector<int> > counter_combinations;
             for(auto item : combinations){
                 vector<int> com;
                 int j = ;
                 for(int i = ; i <= n ; i ++){
                     if(j < item.size() && item[j] == i)
                         j ++;
                     else
                         com.push_back(i);
                 }
                 counter_combinations.push_back(com);
             }
             return counter_combinations;
         }

     }
 };