盡量不要用#define來定義key的文字

應該要改用const NSString以避免可能發生文字覆蓋的錯誤

例如：

.h檔

#import <Foundation/Foundation.h>

extern NSString *SelectionSort;

extern NSString *InsertionSort;

extern NSString *BubbleSort;

@interface Algorithm : NSObject

@end

.m檔

#import "Algorithm.h"

@implementation Algorithm

const NSString *SelectionSort = @"SelectionSort";

const NSString *InsertionSort = @"InsertionSort";

const NSString *BubbleSort    = @"BubbleSort";

@end