public class Solution
extends Object
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:**
**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`