title: Telephone Words problem
body: <p>Please help solve this:</p>
  
  <p>Telephone numbers are often given out as a word representation, so that they are easy to remember. For example if my number is <strong>4357</strong>, the text given is <strong>HELP</strong>. There could be many other possibilities with the same digits, most of which do not make sense.</p>
  
  <p>Write a space-and-time-optimal function that can, given a phone number, print the possible words that can be formed from it.</p>