queue.m

@interface Queue()
    
@property (nonatomic, strong) NSMutableArray<id> *array;

@end
    
@implementation Queue

- (instancetype)init {
    self = [super init];
    
    if (self) {
        _array = [[NSMutableArray alloc] init];
    }
    
    return self;
}

- (void)enqueue:(id)value {
    [self.array addObject:value];
}

- (id)dequeue {
    if (self.array.count > 0) {
        id value = _array[0];
        [self.array removeObjectAtIndex:0];
        return value;
    }
    
    return nil;
}

- (NSString *)description {
    return [NSString stringWithFormat:@"The queue is [%@]", self.array];
}

@end