95\. Unique Binary Search Trees II

Medium

Given an integer `n`, return _all the structurally unique **BST'**s (binary search trees), which has exactly_ `n` _nodes of unique values from_ `1` _to_ `n`. Return the answer in **any order**.

**Example 1:**

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

**Input:** n = 3

**Output:** [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null,null,2],[3,2,null,1]] 

**Example 2:**

**Input:** n = 1

**Output:** [[1]] 

**Constraints:**

*   `1 <= n <= 8`