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

 1 class Solution {

 2 public:

 3     vector<vector<int> > combine(int n, int k) {

 4         vector<vector<int> > combinations;

 5         if(n == 0 || k == 0 || n < k)

 6             return combinations;

 7         if(k == 1){

 8             for(int i = 1; i <= n; i ++) {

 9                 vector<int> com(1, i);

10                 combinations.push_back(com);

11             }

12             return combinations;

13         }

14         

15         if(k == n){

16             vector<int> com;

17             for(int i = 1; i <= n; i ++) {

18                 com.push_back(i);

19             }

20             combinations.push_back(com);

21             return combinations;

22         }

23         

24         if(k <= n / 2){

25             for(int i = 0; i <= n - k; i ++){

26                 if(i == n - k){

27                     vector<int> com;

28                     for(int j = n - k + 1; j <= n; j ++)

29                         com.push_back(j);

30                     combinations.push_back(com);

31                     break;

32                 }

33                 

34                 int pick_num = i + 1;

35                 vector<vector<int> > sub_com = combine(n - pick_num, k - 1);

36                 for(auto item : sub_com) {

37                     for(int j = 0; j < item.size(); j ++){

38                         item[j] += pick_num;                    

39                     }

40                     item.insert(item.begin(), pick_num);

41                     combinations.push_back(item);

42                 }

43             }

44             return combinations; 

45         }else{

46             combinations = combine(n, n - k);

47             vector<vector<int> > counter_combinations;

48             for(auto item : combinations){

49                 vector<int> com; 

50                 int j = 0;

51                 for(int i = 1; i <= n ; i ++){   

52                     if(j < item.size() && item[j] == i)

53                         j ++;

54                     else 

55                         com.push_back(i);

56                 }

57                 counter_combinations.push_back(com);

58             }

59             return counter_combinations;

60         }

61             

62     }

63 };