title: swift sort characters in a string
body: <p>How to sort characters in a string e.g. "5121" -> "1125"  ?</p>
  
  <p>I can do this with code below but it seems too slow:</p>
  
  <pre><code>var nonSortedString = "5121"
  var sortedString = String(Array(nonSortedString.characters).sort())
  </code></pre>