leetcode: 95. Unique Binary Search Trees II [✗]

# Given an integer 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

class TreeNode():
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution():
    def generateTrees(self, n):
        return self.buildTree(0, n)

    def buildTree(self, n1, n2):
        if n1==n2:
            return [None]
        if n1+1 == n2:
            node = TreeNode(n1+1)
            node.left = None
            node.right = None
            return [node]
        else:
            results = []
            for x in range(n1, n2):
                left_list = self.buildTree(n1, x)
                right_list = self.buildTree(x+1, n2)
                for left in left_list:
                    for right in right_list:
                        node = TreeNode(x+1)
                        node.left = left
                        node.right = right
                        results.append(node)
            return results


class TreeNode():
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution():
    def generateTrees(self, n):
        return self.generateTreesRecu(1, n)

    def generateTreesRecu(self, low, high):
        result = []
        if low > high:
            result.append(None)
        for i in range(low, high + 1):
            left = self.generateTreesRecu(low, i - 1)
            right = self.generateTreesRecu(i + 1, high)
            for j in left:
                for k in right:
                    cur = TreeNode(i)
                    cur.left = j
                    cur.right = k
                    result.append(cur)
        return result


if __name__ == "__main__":
    print(Solution().generateTrees(3))