<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*> bst(int left,int right,vector<int>v){
        vector<TreeNode*>ans;
        if(left>right){
            ans.push_back(NULL);
            return ans;
        }
        if(left==right){
            TreeNode* node= new TreeNode(v[left]);
            ans.push_back(node);
            return ans;
        }

        for(int k=left;k<=right;k++){
            vector<TreeNode*>l=bst(left,k-1,v);
            vector<TreeNode*>r=bst(k+1,right,v);
        
            
            for(int i=0;i<l.size();i++)
                for(int j=0;j<r.size();j++){
                    TreeNode* root=new TreeNode(v[k]);
                    root->left=l[i];
                    root->right=r[j];
                    ans.push_back(root);
                }
        }
        return ans;
    }
    vector<TreeNode*> generateTrees(int n) {
        vector<int>v;
        vector<TreeNode*> ans;
        for(int i=1;i<=n;i++)
            v.push_back(i);

        return bst(0,n-1,v);
    }
};
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