def remove_ll_duplicates(root)
  return false unless root
  seen = {}

  current = root
  while current
    if seen[current.val]
      remove_node(current)
    else
      seen[current.val] = true
      current           = current.next
    end
  end
end

def remove_node(node)
  node.val  = node.next.val || nil
  node.next = node.next.next
end