title: How can I print all unused elements?
body: <p>Let's suppose I am holding an array whose size is <code>n</code>, and that I have defined a function called <code>add(i)</code> which increases the value of the element in index <code>i</code> by one.</p>
  <p>How may I print all elements whose value is <code>0</code> (Haven't even been increased) In <code>O(k)</code> where <code>k</code> is the number of those elements?</p>