input: Write a function to check if a binary tree T is a subtree of another binary tree S. If T is a subtree of S return True, else return False. If both T and S are None then return True.
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(3), Node(3))==True
  assert isSubTree(Node(4).left=Node(5), Node(4).left=Node(5))==True
  assert isSubTree(Node(6).left=Node(7), Node(6).right=Node(7))==False
  assert isSubTree(None, None)==True
output: def isSubTree(S, T):
      if T is None:
          return True
      if S is None:
          return False
      
      if isIdentical(S, T):
          return True
      
      return isSubTree(S.left, T) or isSubTree(S.right, T)
  
  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))