Unique Binary Search Tree II ***

 1 /**
 2  * Definition for a binary tree node.
 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         if(n == 0) return generate(1, 0);
14         return generate(1, n);
15     }
16     vector<TreeNode* > generate(int start, int end){
17         vector<TreeNode* > subtree;
18         if(start > end){
19             subtree.push_back(nullptr); //what does it mean?
20             return subtree;
21         }
22         for(int k = start; k <= end; k++){
23             vector<TreeNode* > leftSubtree = generate(start, k - 1);
24             vector<TreeNode* > rightSubtree = generate(k + 1, end);
25             
26             for(int i = 0; i < leftSubtree.size(); i++){
27                 for(int j = 0; j < rightSubtree.size(); j++){
28                     TreeNode* root = new TreeNode(k);
29                     root->left = leftSubtree[i];
30                     root->right = rightSubtree[j];
31                     subtree.push_back(root);
32                 }
33             }
34         }
35         return subtree;
36     }
37 };