input: Write a function to check if a given binary tree is a subtree of another binary tree. If it is a subtree, return True; otherwise, return False. If the given subtree is empty, return True.
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(2), Node(1))==False
  assert isSubTree(Node(3).left = Node(1), Node(3).left = Node(1))==True
  assert isSubTree(Node(4).right = Node(2), Node(4).right = Node(3))==False
  assert isSubTree(None, Node(5))==True
output: def isSubTree(s, t):
      if not s:
          return True
      if not t:
          return False
      if isSameTree(s, t):
          return True
      return isSubTree(s, t.left) or isSubTree(s, t.right)
  
  def isSameTree(s, t):
      if not s and not t:
          return True
      if not s or not t:
          return False
      return s.val == t.val and isSameTree(s.left, t.left) and isSameTree(s.right, t.right)