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, otherwise return False. If the given subtree is empty, return True.
  assert isSubTree(Node(1), Node(None))==False
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(1, Node(2), Node(3)), Node(2))==True
  assert isSubTree(Node(1, Node(2), Node(3)), Node(3))==True
  assert isSubTree(Node(1, Node(2), Node(3)), Node(4))==False
  assert isSubTree(Node(1, Node(2), Node(3)), None)==True
output: def isSubTree(tree, subTree):
      if subTree is None:
          return True
      if tree is None:
          return False
      
      def isSameTree(t1, t2):
          if t1 is None and t2 is None:
              return True
          if t1 is None or t2 is None:
              return False
          return t1.val == t2.val and isSameTree(t1.left, t2.left) and isSameTree(t1.right, t2.right)
      
      if isSameTree(tree, subTree):
          return True
      
      return isSubTree(tree.left, subTree) or isSubTree(tree.right, subTree)