Given an array, remove the duplicates and return a unique array keeping the first occurrence of the duplicates and the order. [@2, @1, @3, @1, @2] --> [@2, @1, @3]

arrayWithoutDuplicates.m

#import <Foundation/Foundation.h>
#import <stdio.h>

@interface Test : NSObject
@end
  
@implementation Test
  
+ (NSArray*)arrayWithoutDuplicates:(NSArray*)a {
  NSMutableSet* s = [NSMutableSet set];
  NSMutableArray* result = [NSMutableArray array];
  for (id obj in a) {
    if (![s containsObject:obj]) {
      [s addObject:obj];
      [result addObject:obj];
    }
  }
  return result;
}
@end

int main (int argc, const char * argv[])
{
  @autoreleasepool {
    
      NSArray* a = @[@2,@1,@3,@1,@2];
      NSArray* b = @[@2,@1,@3];
      NSLog(@"%@", [Test arrayWithoutDuplicates:a]);
      NSLog(@"%d", [[Test arrayWithoutDuplicates:a] isEqualTo:b]);
  }
}

// OUTPUT:
// running 32 lines of Objective-C
// 2014-03-07 07:10:29.856 solution[21] (2, 1, 3)
// 2014-03-07 07:10:29.862 solution[21] 1