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