title: Which number appeared once?
body: <blockquote>
    <p>Given a list of 2n-1 numbers: all between 1 to n, all but one occur twice. Determine the number that occurs only once. Multiple ways preferred. </p>
  </blockquote>
  
  <p>I think the problem is at fault, how can you determine which number without knowing the list of numbers?</p>