[IOS / Object-C] - NSMutableArray 중복값 제거

NSMutableArray *_userIds = [NSMutableArray new];

for( NSInteger i = 0; i < 3; i++ ) {

  [_userIds insertObject:@"aaaaaaaaaa" atIndex:i];

}

NSOrderedSet *userSet = [[NSOrderedSet alloc] initWithArray:_userIds];

_userIds = [[NSMutableArray alloc] initWithArray:[userSet array]];