77. 组合

 1 //直接套模板
 2 class Solution 
 3 {
 4     vector<vector<int>> res;
 5     void helper(vector<int>& nums, int start, vector<int>& out,int k)
 6     {
 7         if(k == out.size()) 
 8         {
 9             res.push_back(out);
10             return;
11         }
12 
13         for (int i = start; i < nums.size(); ++i)
14         {
15             out.push_back(nums[i]);
16             helper(nums, i + 1, out,k);
17             out.pop_back();
18         }
19     }
20 public:
21     vector<vector<int>> combine(int n, int k) 
22     {
23         vector<int> nums,out;
24         for(int i = 1;i <= n;i ++) nums.push_back(i);
25         helper(nums,0,out,k);
26         return res;
27     }
28 };