95. 不同的二叉搜索树 II

 1 class Solution 
 2 {
 3 public:
 4     vector<TreeNode *> generateTrees(int n) 
 5     {
 6         if (n) return generate(1, n);
 7         else return vector<TreeNode *>{};
 8     }
 9     
10     vector<TreeNode *> generate(int left, int right) 
11     {
12         vector<TreeNode *> ans;
13         if (left > right) 
14         {
15             ans.push_back(NULL);
16             return ans;
17         }
18         for (int i = left; i <= right; i++) 
19         {
20             vector<TreeNode *> left_nodes = generate(left, i - 1);
21             vector<TreeNode *> right_nodes = generate(i + 1, right);
22             for (TreeNode *left_node : left_nodes) 
23             {
24                 for (TreeNode *right_node : right_nodes) 
25                 {
26                     TreeNode *t = new TreeNode(i);
27                     t->left = left_node;
28                     t->right = right_node;
29                     ans.push_back(t);
30                 }
31             }
32         }
33         return ans;
34     }
35 };