Combinations

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

 1 class Solution {
 2 public:
 3     vector<vector<int>> combine( int n, int k ) {
 4         vector<int> combination;
 5         DFS( combination, 0, k, n );
 6         return result;
 7     }
 8 private:
 9     void DFS( vector<int> &combination, int i, int k, const int &n ) {
10         if( k == 0 ) { result.push_back( combination ); return; }
11         if( n-i == k ) {
12             vector<int> tmp = combination;
13             for( int val = i+1; val <= n; ++val ) { tmp.push_back( val ); }
14             result.push_back( tmp );
15         } else if( n-i > k ) {
16             DFS( combination, i+1, k, n );
17             combination.push_back( i+1 );
18             DFS( combination, i+1, k-1, n );
19             combination.pop_back();
20         }
21         return;
22     }
23     vector<vector<int>> result;
24 };