<p>Given an integer <code>n</code>, return <em>all the structurally unique <strong>BST&#39;</strong>s (binary search trees), which has exactly </em><code>n</code><em> nodes of unique values from</em> <code>1</code> <em>to</em> <code>n</code>. Return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/01/18/uniquebstn3.jpg" style="width: 600px; height: 148px;" />
<pre>
<strong>Input:</strong> n = 3
<strong>Output:</strong> [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> [[1]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 8</code></li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
vector<TreeNode*> res;
TreeNode* makeTree(TreeNode* root){
    if(!root) return NULL;
    TreeNode* c = new TreeNode(root->val);
    c->left = makeTree(root->left);
    c->right = makeTree(root->right);
    return c;
}
void add(TreeNode* root, int n){
    TreeNode* tmp = new TreeNode(n);
    tmp->left = root;
    res.push_back(makeTree(tmp));
    tmp->left = NULL;
    TreeNode* node = root;
    while(node){
        TreeNode* r = node->right;
        node->right = tmp;
        tmp->left = r;
        res.push_back(makeTree(root));
        node->right = r;
        tmp->left = NULL;
        node = node->right;
    }
}
    vector<TreeNode*> generateTrees(int n) {
        if(n == 1){
            TreeNode* root = new TreeNode(1);
            res.push_back(root);
            return res;
        }
        vector<TreeNode*> pre = generateTrees(n - 1);
        vector<TreeNode*> v;
        res = v;
        for(int i = 0; i < pre.size(); i++){
            add(pre[i], n);
        }
        return res;
    }
};
19100