BST.py

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

class Solution:
  def isValidBST(self, root):
    def helper(node, lower, upper):
      if not node:
        return True
      val = node.val
      if val <= lower or val >= upper:
        return False
      if not helper(node.right, val, upper):
        return False
      if not helper(node.left, lower, val):
        return False
      return True
    return helper(root, float('-inf'), float('inf'))

node = TreeNode(5)
node.left = TreeNode(4)
node.right = TreeNode(7)
print(Solution().isValidBST(node))
# True

"""

class Solution:

	def isValidBST(self, root):

		def helper(node, left ,right):

			if not node:

				return True

			if not left < val < right:

				return False

			return helper(node.left, left, node.val) and helper(node.right, node.val, right)

		return helper(root, float("-inf"), float("inf"))



"""