Input1:      5    / \   6   7  /   / \ 2   4   9Output1: 749// The largest binary search tree is .  7 / \4   9

#!/usr/bin/env python3# -*- coding: utf-8 -*-class TreeNode:  def __init__(self, value):    self.left = None    self.right = None    self.value = value
  def __str__(self):    The forward sequence traverses the output binary tree.    answer = str(self.value)    if self.left:      answer += str(self.left)    if self.right:      answer += str(self.right)    return answer
class Solution:  def largestBSTSubtree(self, root):    self.maxSize = 0    self.maxRoot = None
    def helper(root):      Returns information that a metagroup (size, minValue, maxValue) represents the corresponding sub-tree for the current node.      if root is None:        return (0, float('inf'), float('-inf'))      left = helper(root.left)      right = helper(root.right)      if root.value > left[2] and root.value < right[1]:        size = left[0] + right[0] + 1        if size > self.maxSize:          self.maxSize = size          self.maxRoot = root        return (size, min(root.value, left[1]), max(root.value, right[2]))      else:        return (0, float('-inf'), float('inf'))
    helper(root)    return self.maxRoot
# Test Program#     5#    / \#   6   7#  /   / \# 2   4   9node = TreeNode(5)node.left = TreeNode(6)node.right = TreeNode(7)node.left.left = TreeNode(2)node.right.left = TreeNode(4)node.right.right = TreeNode(9)print(Solution().largestBSTSubtree(node))# 749