input: Write a function to check if the first binary tree is a subtree of the second binary tree. If the first tree is a subtree, return True; otherwise, return False. If the first tree is empty, return True.
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(2), Node(3))==False
  assert isSubTree(None, Node(4))==True
  assert isSubTree(Node(5), None)==False
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)