input: Write a function to determine 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 T or S is None, return False.
  assert isSubTree(Node(3), Node(3))==True
  assert isSubTree(Node(3), Node(5))==False
  assert isSubTree(Node(3), None)==False
  assert isSubTree(None, Node(5))==False
output: def isSubTree(T, S):
      if S is None:
          return False
      if T is None:
          return False
      if isIdentical(T, S):
          return True
      return isSubTree(T.left, S) or isSubTree(T.right, S)
  
  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))