validate_binary_search_tree.py

# 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

# define a method to test if it is a BST
defBST_valid(root, min_value, max_value):

# not a TreeNode
ifnotroot:
returnTrue

# the cases: not a BST
if (root.val>=max_value) or (root.val<=min_value):
returnFalse

# test the left node
left_test=BST_valid( root.left, min_value, root.val)
# test the right node
right_test=BST_valid( root.right, root.val, max_value)

# return the result
return (left_testandright_test)


classSolution:
defisValidBST(self, root: TreeNode) ->bool:

# initialize the max and min values
max_inf=float('inf')
min_inf=float('-inf')

# recursive test
returnBST_valid(root, min_inf, max_inf)