输入参数是NSDate，输出结果是星期几的字符串

给你一个方法，输入参数是NSDate，输出结果是星期几的字符串。
+ (NSString*)weekdayStringFromDate:(NSDate*)inputDate {

NSArray *weekdays = [NSArray arrayWithObjects: [NSNull null], @"Sunday", @"周一", @"周二", @"周三", @"周四", @"周五", @"周六", nil];

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSTimeZone *timeZone = [[NSTimeZone alloc] initWithName:@"Asia/Shanghai"];

[calendar setTimeZone: timeZone];

NSCalendarUnit calendarUnit = NSWeekdayCalendarUnit;

NSDateComponents *theComponents = [calendar components:calendarUnit fromDate:inputDate];

return [weekdays objectAtIndex:theComponents.weekday];

}