## Unique Binary Search Trees

Given `n`, how many structurally unique **BST's** (binary search trees) that store values 1 ... `n`?

### Examples

```text
Input: 3
Output: 5
Explanation:
Given n = 3, there are a total of 5 unique BST's:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3
```

### Constraints

- `1 <= n <= 19`

---

### Notes

- N/A