<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:
  vector<TreeNode*> generateTrees(int n) {
    if (n == 0)
      return {};
    return generateTrees(1, n);
  }

 private:
  vector<TreeNode*> generateTrees(int min, int max) {
    if (min > max)
      return {nullptr};

    vector<TreeNode*> ans;

    for (int i = min; i <= max; ++i)
      for (TreeNode* left : generateTrees(min, i - 1))
        for (TreeNode* right : generateTrees(i + 1, max)) {
          ans.push_back(new TreeNode(i));
          ans.back()->left = left;
          ans.back()->right = right;
        }

    return ans;
  }
};
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