去掉数组中相同的元素

NSMutableArray *namearray = [NSMutableArray arrayWithObjects:@"I am a picture", @"I am a guy", @"I am a picture", @"ipad", @"iphone", nil];

    NSMutableArray *categoryArray = [[NSMutableArray alloc] init];

    for (unsigned i = 0; i < [namearray count]; i++){

        if ([categoryArray containsObject:[namearray objectAtIndex:i]] == NO){

            [categoryArray addObject:[namearray objectAtIndex:i]];

        }

    }