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 given subtree is empty then return True.
  assert isSubTree(Node(5), Node(5)) == True
  assert isSubTree(Node(5), Node(7)) == False
  assert isSubTree(Node(5), None) == True
  assert isSubTree(Node(5), Node(5, left=Node(3), right=Node(8))) == False
  assert isSubTree(None, None) == True
output: def isSubTree(tree, subTree):
      if subTree is None:
          return True
      if tree is None:
          return False
      if isIdentical(tree, subTree):
          return True
      return isSubTree(tree.left, subTree) or isSubTree(tree.right, subTree)
  
  def isIdentical(node1, node2):
      if node1 is None and node2 is None:
          return True
      if node1 is None or node2 is None:
          return False
      return (node1.val == node2.val and 
              isIdentical(node1.left, node2.left) and 
              isIdentical(node1.right, node2.right))