<p>Given an integer <code>n</code>, return <em>the number of 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>.</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> 5
</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;= 19</code></li>
</ul>

class Solution:
    def numTrees(self, n: int) -> int:
        dp = [1]*(n+1)

        for i in range(2, n+1):
            sum = 0
            for j in range(i):
                sum += dp[j]*dp[i-j-1]
            dp[i] = sum
        
        return dp[-1]
        
16500