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 == 0 || k == 0 || n < k)
            return combinations;
        if(k == 1){
            for(int i = 1; i <= n; i ++) {
                vector<int> com(1, i);
                combinations.push_back(com);
            }
            return combinations;
        }
        
        if(k == n){
            vector<int> com;
            for(int i = 1; i <= n; i ++) {
                com.push_back(i);
            }
            combinations.push_back(com);
            return combinations;
        }
        
        if(k <= n / 2){
            for(int i = 0; i <= n - k; i ++){
                if(i == n - k){
                    vector<int> com;
                    for(int j = n - k + 1; j <= n; j ++)
                        com.push_back(j);
                    combinations.push_back(com);
                    break;
                }
                
                int pick_num = i + 1;
                vector<vector<int> > sub_com = combine(n - pick_num, k - 1);
                for(auto item : sub_com) {
                    for(int j = 0; 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 = 0;
                for(int i = 1; i <= n ; i ++){   
                    if(j < item.size() && item[j] == i)
                        j ++;
                    else 
                        com.push_back(i);
                }
                counter_combinations.push_back(com);
            }
            return counter_combinations;
        }
            
    }
};