这一天是星期几（难度：1颗星）

问题描述：

输入1个日期，日期的格式由3个正整数组成（正整数之间用空格分开），输出这一天是星期几，如果日期不合法，比如：2008 1 32（这个就不合法，因为天数超过了1月的最大天数31），直接输出“输入日期无效”，否则输出这一天是星期几。

输入举例：

2017 1 1

输出结果：
星期天

参考代码：

#include <stdio.h>int arr[] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };char szDays[7][5] = {"天", "一", "二", "三", "四", "五", "六"};int IsLeapYear(int year){    return ((year % 4 == 0) && (year % 100)) || (year % 400 == 0);}int GetPassedMonthDays(int year, int month, int day){//计算因为月份相差的天数    int sum = day;    for (int i = 1; i < month; i++)        sum += arr[i];    if (month > 2 && IsLeapYear(year))        sum += 1;    return sum;}int GetDiffYearDays(int year1, int year2){//计算因为年份，相差的天数    int sum = (year2 - year1) * 365;    for (int i = year1; i < year2; i++)    {        if (IsLeapYear(i))            sum += 1;    }    return sum;}int isvalid(int year, int month, int day){//判断输入的日期是否有效    if (month < 0 || month > 12)        return 0;    if (day < 0)        return 0;    if (month == 2)    {        if ((IsLeapYear(year) && day > 29) || (!IsLeapYear(year) && day > 28))            return 0;    }    else    {        if (day > arr[month])            return 0;    }    return 1;}int IsGreater(int year1, int month1, int day1, int year2, int month2, int day2){//判断两个日期那个更大    if (year1 != year2)        return year1 > year2 ? 1 : 0;    if (month1 != month2)        return month1 > month2 ? 1 : 0;    if (day1 != day2)        return day1 > day2 ? 1 : 0;    return 0;}int main(){    int year1, month1, day1, dayOfWeek1, nDayDiff;    int year2 = 2017, month2 = 5, day2 = 27, dayOfWeek2 = 6;//以今天的时间为基准，2017-5-27，星期6    printf("请输入日期的年月日,用空格隔开\n");    scanf_s("%d%d%d", &year1, &month1, &day1);    if (!isvalid(year1, month1, day1))    {        printf("输入日期无效\n");        return 0;    }    if (IsGreater(year1, month1, day1, year2, month2, day2))    {        nDayDiff = GetDiffYearDays(year2, year1) - GetPassedMonthDays(year2, month2, day2) + GetPassedMonthDays(year1, month1, day1);        dayOfWeek1 = (dayOfWeek2 + nDayDiff) % 7;    }    else    {        nDayDiff = GetDiffYearDays(year1, year2) - GetPassedMonthDays(year1, month1, day1) + GetPassedMonthDays(year2, month2, day2);        dayOfWeek1 = ((dayOfWeek2 - nDayDiff) % 7 + 7) % 7;    }    printf("星期%s\n", szDays[dayOfWeek1]);    return 0;}

运行结果：