[leetcode]Unique Binary Search Trees II

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<TreeNode*> make(int start , int end) {
vector<TreeNode*>result;
if(start > end) {
result.push_back(nullptr);
return result;
}
for(int i = start ; i <= end ; ++i) {
auto leftsub = make(start , i - 1);
auto rightsub = make(i + 1 , end);
for(auto left : leftsub) {
for(auto right : rightsub) {
TreeNode* root = new TreeNode(i);
root->left = left;
root->right = right;
result.push_back(root);
}
}
}
return result;
}
vector<TreeNode *> generateTrees(int n) {
return make(1 , n);
}
};