Difficulty: Medium
Tags: LeetCode, Medium, Tree, Binary Search Tree, Dynamic Programming, Backtracking, Binary Tree, leetcode-95, Recursion, O(4^n / n^(3/2), LintCode, O(n * 4^n / n^(3/2)), O(n)

Problem

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