Check if given Binary Tree is Binary Search Tree (BST) or not.

isValidBST.py

class TreeNode:
  def __init__(self, value, left=None, right=None):
    self.value = value
    self.left = left
    self.right = right
  
  def __str__(self):
    return "(%s)"% (self.value)


def isValidBST(root, min ,max):
  if root is None:
    return True
  if root.value <= min or root.value > max:
    return False
  return isValidBST(root.left, min, root.value) and isValidBST(root.right, root.value, max)


t = TreeNode(10)
t.left = TreeNode(10)
t.left.left = TreeNode(-5)
t.right = TreeNode(19)
t.right.left = TreeNode(17)
t.right.right = TreeNode(21)

print(isValidBST(t, float('-inf') , float('inf')))


t1 = TreeNode(10)
t1.left = TreeNode(0)
t1.left.left = TreeNode(-1)
t1.left.right = TreeNode(21)
t1.right = TreeNode(25)
t1.right.left = TreeNode(16)
t1.right.right = TreeNode(32)

print(isValidBST(t1, float('-inf'), float('inf')))