input: Write a function to determine if a given binary tree is a subtree of another binary tree. If it is a subtree return True else return False. If the subtree is None then return True.
  
  class Node:
      def __init__(self, data):
          self.data = data
          self.left = None
          self.right = None
  
  def isSubTree(T, S):
      if S is None:
          return True
      if T is None:
          return False
      if areIdentical(T, S):
          return True
      return isSubTree(T.left, S) or isSubTree(T.right, S)
  
  def areIdentical(T, S):
      if T is None and S is None:
          return True
      if T is None or S is None:
          return False
      return (T.data == S.data and areIdentical(T.left, S.left)and
          areIdentical(T.right, S.right))
  
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(1), None)==True
  assert isSubTree(None, Node(1))==False
  assert isSubTree(Node(1, Node(2), Node(3)), Node(2))==True
output: def isSubTree(T, S):
      if S is None:
          return True
      if T is None:
          return False
      if areIdentical(T, S):
          return True
      return isSubTree(T.left, S) or isSubTree(T.right, S)
  
  def areIdentical(T, S):
      if T is None and S is None:
          return True
      if T is None or S is None:
          return False
      return (T.data == S.data and areIdentical(T.left, S.left) and
          areIdentical(T.right, S.right))