leetcode 95. Unique Binary Search Trees II

 1 vector<TreeNode *> generateTrees(int n) 
 2     {
 3         return BSTGenerate(1, n);
 4     }
 5     
 6     vector<TreeNode *> BSTGenerate(int start, int end)
 7     {
 8         TreeNode *root = NULL;
 9         vector<TreeNode *> left_trees, right_trees, trees;
10         if (start > end)
11         {
12             trees.push_back(NULL);
13             return trees;
14         }
15         
16         for (int i = start; i <= end; i++)
17         {
18             left_trees =  BSTGenerate(start, i - 1);
19             right_trees = BSTGenerate(i + 1, end);
20             for (int j = 0; j < left_trees.size(); j++)
21             {
22                 for (int k = 0; k < right_trees.size(); k++)
23                 {
24                     root = new TreeNode(i);
25                     root->left = left_trees[j];
26                     root->right = right_trees[k];
27                     trees.push_back(root);
28                 }
29             }   
30         }
31         
32         return trees;
33     }