<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 {
    int solve(int n){
        vector<int> dp(n + 1,0);
        dp[0] = dp[1] = 1;

        for(int i = 2; i <= n;i++){
            for(int j = 1; j <= i;j++){
                dp[i] += dp[j - 1] * dp[i - j]; 
            }
        }
        return dp[n];
    }
public:
    int numTrees(int n) {
        return solve(n);
    }
};
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