input: Write a function to check if a given binary tree S is a subtree of another binary tree T. If S is a subtree of T, return True, otherwise return False. If either of the trees is empty, return True only if S is empty.
  assert isSubTree(None, None)==True
  assert isSubTree(Node(1), None)==True
  assert isSubTree(None, Node(2))==False
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(1, Node(2), Node(3)), Node(2))==True
  assert isSubTree(Node(1, Node(2), Node(3)), Node(3, Node(4)))==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(root1, root2):
      if root1 is None and root2 is None:
          return True
      if root1 is None or root2 is None:
          return False
      return (root1.val == root2.val and 
              isIdentical(root1.left, root2.left) and 
              isIdentical(root1.right, root2.right))