title: Objective-C Dictionary keys sorting in descending order
body: <p>I have an unordered dictionary like this:</p>
  
  <pre><code>@{ @"20101R" : @"value1", 
     @"20102R" : @"value2", 
     @"20131R" : @"value3", 
     @"20111R" : @"value4", 
     @"20122R" : @"value5" }
  </code></pre>
  
  <p>Each key represents <strong>Year</strong> and either <strong>1R</strong> or <strong>2R</strong>.</p>
  
  <p>For UITableView, I wish to put 'keys' as title and 'values' as subtitle like this:</p>
  
  <p>from <strong>latest</strong> key value -> <strong>oldest</strong> key value </p>
  
  <pre><code>title : 20131R
  subtitle : value3
  
  title : 20122R
  subtitle : value5
  
  title : 20111R
  subtitle : value4
  
  title : 20102R
  subtitle : value2
  
  title : 20101R
  subtitle : value1
  </code></pre>
  
  <p>Can you help me sort this number-like string in descending order?
  What is the simplest way to do this? 
  Thank you in advance.</p>