Testing whether a binary tree is a binary search three: class BTNode(object): def __init__(self, value, left=None, right=None): self.value = value self.left = left self.right = right def BSTMin(node): return BSTMin(node.left) if node.left else node.value def BSTMax(node): return BSTMax(node.right) if node.right else node.value def isBST(node): if node.left and not (isBST(node.left) and BSTMax(node.left) < node.value): return False if node.right and not (isBST(node.right) and node.value <= BSTMin(node.right)): return False return True