title: how to sort array of strings
body: <p>I create an array with string names as shown below</p>
  
  <pre><code>NSMutableArray *strings = [[NSMutableArray alloc]init];
  [string addObject:@"string1"];
  [string addObject:@"string2"];
  [string addObject:@"string3"];
  [string addObject:@"string4"];
  </code></pre>
  
  <p>and I create a button. Whenever I click the button the strings are exchanged how can I do this?</p>