Combinations

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],
]

思路：
DFS
另外，用next permutation里提到的方法也可以。
代码：

 1     void search(int start, int n, int k, vector<vector<int> > &result, vector<int> &tmp){
 2         if(k == 0){
 3             if(start == 0)
 4                 return;
 5             result.push_back(tmp);
 6             return;
 7         }
 8 
 9         if(start > n || n - start + 1 < k)
10             return;
11         search(start+1, n, k, result, tmp);
12         tmp.push_back(start);
13         search(start+1, n, k-1, result, tmp);
14         tmp.pop_back();
15     }
16     vector<vector<int> > combine(int n, int k) {
17         // IMPORTANT: Please reset any member data you declared, as
18         // the same Solution instance will be reused for each test case.
19         vector<vector<int> > result;
20         vector<int> tmp;
21         if(k == 0)  
22             return result;
23         search(1, n, k, result, tmp);
24         return result;
25     }