title: Check if there's a duplicate in an array using an hash-table
body: <blockquote>
    <p>Goal: Check if there's a duplicate number in an array with the size of <code>n</code>.</p>
  </blockquote>
  
  <p>Basically if we may use an hash-table (open-hash, with linked list), then we could iterate the array and insert the numbers to the table with some value (could be <code>1</code>, doesn't really matter).
  While iterating, if the cell isn't empty then we have a duplicate number.</p>
  
  <p>Since we know that the <strong>expected</strong> time for read/write is <code>O(1)</code> then the expected time for the algorithm is <code>O(n)</code>.</p>
  
  <p><strong>Question #1:</strong> Why is the worst-case equal <code>O(nlogn)</code>?<br>
  <strong>Question #2:</strong> Would you do it differently then the solution suggested?</p>