title: get distinct value from NSmutablearray
body: <p>i have a <code>nsmutablearray</code> as is shown below. I would like to retrieve list value without redundancy. in this example i should retrive <code>10 20 30</code></p>
  
  <pre><code>NSMutableArray *array = [[NSMutableArray alloc]initWithObjects:@"10",@"20",@"30",@"30",@"30",@"30",@"20", nil];
  </code></pre>