title: How to increment String in Swift
body: <p>I need to save files in an alphabetical order. <br>
  Now my code is saving files in numeric order</p>
  
  <blockquote>
    <p>1.png<br>
    2.png<br>
    3.png<br> ...<br></p>
  </blockquote>
  
  <p>The problem is when i read this files again I read this files as described <a href="https://stackoverflow.com/questions/38819395/order-a-nsurl-array">here</a><br></p>
  
  <p><br>So I was thinking of changing the code and to save the files not in a numeric order but in an <strong>alphabetical order</strong> as:</p>
  
  <blockquote>
    <p>a.png<br> b.png<br> c.png<br> ...<br> z.png<br> aa.png<br> ab.png<br> ...</p>
  </blockquote>
  
  <p>But in Swift it's difficult to increment even <em>Character</em> type.<br>
  How can I start from: </p>
  
  <pre><code>var s: String = "a"
  </code></pre>
  
  <p>and increment <em>s</em> in that way?</p>