96\. Unique Binary Search Trees

Medium

Given an integer `n`, return _the number of structurally unique **BST'**s (binary search trees) which has exactly_ `n` _nodes of unique values from_ `1` _to_ `n`.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/01/18/uniquebstn3.jpg)

**Input:** n = 3

**Output:** 5 

**Example 2:**

**Input:** n = 1

**Output:** 1 

**Constraints:**

*   `1 <= n <= 19`