input: Write a function to determine if two binary trees are similar by comparing their structures and values at each node. If both trees are similar return True else return False. If either or both of the input roots are None then return True only if both are None.
  
  assert are_similar(None, None)==True
  assert are_similar(TreeNode(1), None)==False
  assert are_similar(None, TreeNode(1))==False
  assert are_similar(TreeNode(1), TreeNode(1))==True
  assert are_similar(TreeNode(1, TreeNode(2), TreeNode(3)), TreeNode(1, TreeNode(2), TreeNode(3)))==True
  assert are_similar(TreeNode(1, TreeNode(2), TreeNode(3)), TreeNode(1, TreeNode(3), TreeNode(2)))==False
output: def are_similar(root1, root2):
      if root1 is None and root2 is None:
          return True
      if root1 is None or root2 is None:
          return False
      if root1.val != root2.val:
          return False
      return are_similar(root1.left, root2.left) and are_similar(root1.right, root2.right)