Leetcode (Python): Unique Binary Search Trees II

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

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

Solution:

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @return a list of tree node
    def generateTrees(self, n):
        return self.generateTrees2(1,n)
    
    def generateTrees2(self, begin, end):
        solution = []
        if begin > end:
            solution.append(None)
            return solution
        for root in range(begin,end+1):
            leftNodes = self.generateTrees2(begin, root-1)
            rightNodes = self.generateTrees2(root+1, end)
            for leftNode in leftNodes:
                for rightNode in rightNodes:
                    node = TreeNode(root)
                    node.left = leftNode
                    node.right = rightNode
                    solution.append(node)
        return solution