Unique Binary Search Trees

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:** **Input:** n = 3 **Output:** 5 ``` **Example 2:** **Input:** n = 1 **Output:** 1 ```   **Constraints:** - `1 <= n <= 19`