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# Time: O(n)

# Space: O(h)

# Definition for a binary tree node.

# class TreeNode(object):

# def __init__(self, x):

# self.val = x

# self.left = None

# self.right = None

class Solution(object):

def largestBSTSubtree(self, root):

"""

:type root: TreeNode

:rtype: int

"""

if root is None:

return 0

max_size = [1]

def largestBSTSubtreeHelper(root):

if root.left is None and root.right is None:

return 1, root.val, root.val

left_size, left_min, left_max = 0, root.val, root.val

if root.left is not None:

left_size, left_min, left_max = largestBSTSubtreeHelper(root.left)

right_size, right_min, right_max = 0, root.val, root.val

if root.right is not None:

right_size, right_min, right_max = largestBSTSubtreeHelper(root.right)

size = 0

if (root.left is None or left_size > 0) and \

(root.right is None or right_size > 0) and \

left_max <= root.val <= right_min:

size = 1 + left_size + right_size

max_size[0] = max(max_size[0], size)

return size, left_min, right_max

largestBSTSubtreeHelper(root)

return max_size[0]