title: How to find the only number in an array that doesn't occur twice
body: <p>The following is taken from a job interview: </p>
  
  <blockquote>
    <p>In a given array, which contains integers, each number repeats itself
    once except for one, which doesn't repeat. Write a function that finds
    the number that doesn't repeat.</p>
  </blockquote>
  
  <p>I thought about using an HashSet, but it might complicate everything...</p>
  
  <p>Any ideas of a simple solution?</p>