title: NSMutableArray add object with order
body: <p>I have a <code>NSMUtableArray</code> which has elements, for example:</p>
  
  <pre>
  a,b,c,e
  </pre>
  
  <p>And I want to add an object <code>d</code> to behind <code>c</code> and before <code>e</code>.
  In other words, I'd like to insert an object to a sorted array.(The object can be a custom object, too)</p>
  
  <p>I'd like to know : besides using <code>for</code> to find the position, is there any other method to implement it? It is better to use the iOS api.</p>
  
  <p>Thanks.</p>