LeetCode-- Unique Binary Search Trees II

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<TreeNode *> generateTrees(int n) {
13         return dfs(1,n);
14     }
15     vector<TreeNode *> dfs(int l,int r)
16     {
17         vector<TreeNode *> res;
18         if(l > r)
19         {
20             res.push_back(NULL);
21             return res;
22         }
23         vector<TreeNode *> left;
24         vector<TreeNode *> right;
25         for(int cur = l ; cur <= r  ; ++cur)
26         {
27             left = dfs(l,cur-1);
28             right = dfs(cur+1,r);
29             for(int i = 0 ; i < left.size() ; ++i)
30             {
31                 for(int j = 0 ; j < right.size() ; ++j)
32                 {
33                     TreeNode *root = new TreeNode(cur);
34                     root->left = left[i];
35                     root->right = right[j];
36                     res.push_back(root);
37                 }
38             }
39         }
40         return res;
41     }
42 };