input: Write a function to check if one binary tree is a subtree of another binary tree. If it is a subtree, return True, otherwise return False.
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(1, left=Node(2), right=Node(3)), Node(2))==True
  assert isSubTree(Node(1, left=Node(2), right=Node(3)), Node(3))==True
  assert isSubTree(Node(1, left=Node(2), right=Node(3)), Node(4))==False
  assert isSubTree(None, None)==True
output: def isSubTree(root1, root2):
      if not root2:
          return True
      if not root1:
          return False
      if isIdentical(root1, root2):
          return True
      return isSubTree(root1.left, root2) or isSubTree(root1.right, root2)
  
  def isIdentical(node1, node2):
      if not node1 and not node2:
          return True
      if not node1 or not node2:
          return False
      return (node1.val == node2.val and 
              isIdentical(node1.left, node2.left) and 
              isIdentical(node1.right, node2.right))