<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>

class Solution {
public:
    TreeNode * build(TreeNode * root){
        if(root==NULL){
            return NULL;
        }
        TreeNode * temp=new TreeNode (root->val);
        temp->left=build(root->left);
        temp->right=build(root->right);
        return temp;
    }
    vector<TreeNode*> generateTrees(int n) {
        if(n==1){
            return {new TreeNode(1)};
        }
        unordered_map<int,vector<TreeNode *>>mp;
        mp[1].push_back(new TreeNode(1));
        for(int i=2;i<=n;i++){
            for(auto it : mp[i-1]){
                
                TreeNode * top=new TreeNode(i);

                TreeNode * less=build(it);
                top->left=less;

                mp[i].push_back(top);

                TreeNode * mid=build(it);
                TreeNode * node=mid;
                
                while(mid!=NULL){
                    TreeNode * right=build(mid->right);
                    mid->right=new TreeNode(i);
                    mid->right->left=right;
                    mp[i].push_back(build(node));
                    mid->right=right;
                    mid=mid->right;
                } 
                
            }
        }
        return mp[n];
    }
};
19300