iOS 实现一个栈Stack

栈的基本操作: push, pop, peek
代码来自appledoc的开源项目

用拓展来处理，非常方便，不用专门定义一个Stack.

@implementation NSArray (GBArray)

- (id)firstObject {
    if ([self count] == 0) return nil;
    return self[0];
}

- (BOOL)isEmpty {
    return ([self count] == 0);
}
#pragma mark -

@implementation NSMutableArray (GBMutableArray)

- (void)push:(id)object {
    [self addObject:object];
}

- (id)pop {
    id result = [self peek];
    [self removeLastObject];
    return result;
}

- (id)peek {
    return [self lastObject];
}

@end