input: Write a python function to detect if a given tree is a binary search tree.
  assert isBST(Node(5)) == True
  assert isBST(None) == True
  root = Node(10)
  root.left = Node(5)
  root.right = Node(15)
  assert isBST(root) == True
  root.right.left = Node(6)
  assert isBST(root) == False
output: class Node:
      def __init__(self, key):
          self.data = key
          self.left = None
          self.right = None
  
  def isBST(node, left=float('-inf'), right=float('inf')):
      if node is None:
          return True
      if not (left < node.data < right):
          return False
      return isBST(node.left, left, node.data) and isBST(node.right, node.data, right)