<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:
    TreeNode* transform(TreeNode* root, int i) {
        if (root) {
            TreeNode* left = transform(root->left, i);
            TreeNode* right = transform(root->right, i);
            TreeNode* temp = new TreeNode(root->val + i);
            temp->left = left;
            temp->right = right;
            return temp;
        }
        return nullptr;
    }
    vector<TreeNode*> generateTrees(int n) {
        map<int, vector<TreeNode*>> mpp;
        mpp[0].push_back(nullptr);
        mpp[1].push_back(new TreeNode(1));

        for (int i = 2; i <= n; i++) {
            for (int j = 1; j <= i; j++) {
                int left = j - 1;
                int right = i - j;
                for (auto& leftPtr : mpp[left]) {
                    for (auto& rightPtr : mpp[right]) {
                        TreeNode* root = new TreeNode(j);
                        root->left = transform(leftPtr, 0);
                        root->right = transform(rightPtr, j);
                        mpp[i].push_back(root);
                    }
                }
            }
        }
        return mpp[n];
    }
};
18100