# [leetcode]Unique Binary Search Trees II

2013-07-12

Given n, generate all structurally unique BST&#39;s (binary search trees) that store values 1...n.For example,Given n = 3, your program should return all 5 unique BST&#39;...

Given n, generate all structurally unique BST&#39;s (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST&#39;s shown below.

1 3 3 2 1
\ / / / \ \
3 2 1 1 3 2
/ / \ \
2 1 2 3

```/**
* 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 *> generateSub(int N, int start){
if(N == 0){
//   return vector<TreeNode *>(1, NULL);
vector<TreeNode*> tmp;
tmp.push_back(NULL);
return tmp;
}

vector<TreeNode *> curLeft, curRight, result;
for(int n = 1; n <= N; n++){
int rootval = n + start - 1;

curLeft = generateSub(n-1, start);
curRight = generateSub(N-n, rootval+1);

for(int i = 0; i < curLeft.size(); i++){
for(int j = 0; j < curRight.size(); j++){
TreeNode *root = new TreeNode(rootval);
root -> left = curLeft[i];
root -> right = curRight[j];

result.push_back(root);
}
}
}

return result;
}

vector<TreeNode *> generateTrees(int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
return generateSub(n, 1);

}
};```