title: Find longest substring without repeating characters
body: <p>Given a <code>string S</code> of <code>length N</code> find longest substring without repeating characters.</p>
  
  <p><strong>Example:</strong> </p>
  
  <p><strong>Input:</strong> "stackoverflow"</p>
  
  <p><strong>Output:</strong> "stackoverfl"</p>
  
  <p>If there are two such candidates, return first from left. I need linear time and constant space algorithm.</p>