<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*> gen(int start,int end){
        if(start>end){
            return {};
        }
        if(start==end){
            vector<TreeNode*> arr;
            TreeNode* tempp=new TreeNode(start);
            arr.push_back(tempp);
            return arr;
        }
        
        vector<TreeNode*> give;
        for(int ii=start;ii<=end;ii++){
            vector<TreeNode*>left;
        vector<TreeNode*>right;
            left=gen(start,ii-1);
            right=gen(ii+1,end);
            if(left.size()>0 and right.size()==0){
                right.push_back(NULL);
              }
            if(right.size()>0 and left.size()==0){
                left.push_back(NULL);
            }

           
            for(int i=0;i<left.size();i++){
                for(int j=0;j<right.size();j++){
                        TreeNode* curr=new TreeNode(ii);
                        curr->left=left[i];
                        curr->right=right[j];
                        give.push_back(curr);
                }
            }
        }
        return give;
    }
    vector<TreeNode*> generateTrees(int n) {
        return gen(1,n);

    }
};
19500