input: Write a function to check if a binary tree is a subtree of another binary tree. The function should return True if the first tree is a subtree of the second tree, otherwise it should return False. If both trees are None, the function should return True.
  
  assert isSubTree(None, None)==True
  assert isSubTree(Node(1), None)==False
  assert isSubTree(None, Node(1))==True
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(1, Node(2), Node(3)), Node(3))==True
  assert isSubTree(Node(1, Node(2), Node(3)), Node(1, Node(2), Node(3)))==True
output: def isSubTree(s, t):
      if s is None:
          return True
      if t is None:
          return False
      if isIdentical(s, t):
          return True
      return isSubTree(s, t.left) or isSubTree(s, t.right)
  
  def isIdentical(s, t):
      if s is None and t is None:
          return True
      if s is None or t is None:
          return False
      return s.val == t.val and isIdentical(s.left, t.left) and isIdentical(s.right, t.right)