title: Efficiently selecting a set of random elements from a linked list
body: <p>Say I have a linked list of numbers of length <code>N</code>. <code>N</code> is very large and I don’t know in advance the exact value of <code>N</code>. </p>
  
  <p>How can I most efficiently write a function that will return <code>k</code> completely <em>random numbers</em> from the list?</p>