input: Write a function to check if a binary tree is a subtree of another binary tree. If the second tree contains the first tree as a subtree, return True; otherwise, return False. If the first tree is an empty tree, return True.
  def isSubTree(T, S):
  
  assert isSubTree(Node(10), Node(10))==True
  assert isSubTree(Node(5), Node(10, Node(5), Node(15)))==True
  assert isSubTree(Node(5, None, Node(10)), Node(10))==False
  assert isSubTree(Node(4, Node(3), Node(6)), Node(2, Node(4, Node(3), Node(6)), Node(5)))==True
  assert isSubTree(None, Node(10))==True
output: def isSubTree(T, S):
      if T is None:
          return True
      if S is None:
          return False
      if isIdentical(T, S):
          return True
      return isSubTree(T, S.left) or isSubTree(T, S.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))