[LeetCode] 95. Unique Binary Search Trees II

Problem : https://leetcode.com/problems/unique-binary-search-trees-ii/

In BST,  each node is larger than every nodes in its left sub-tree and smaller than every nodes in its right sub-tree.  We should use every valid integer as root node and create BST recursively.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def generateTrees(self, n: int) -> List[Optional[TreeNode]]:
        
        def helper(left, right):
            if left == right:
                return [TreeNode(val=left)]
            elif left > right:
                return [None]
            else:
                return [TreeNode(val=pivot, left=l, right=r) for pivot in range(left, right+1) for l in helper(left, pivot - 1) for r in helper(pivot+1, right)]

        return helper(1, n)

Edited on 09/02/2021. Update a simpler solution.