在1970-9999年之間輸入一個日期實現輸出星期幾

程序很簡單就直接貼代碼：

#include<stdio.h>
int main()
{
	int year;
	int month;
	int day;
	int week;
	long int x;
	int y;
	int z;
	int n;
	long int p;
	int sum=0;
	int i=1970;
	int  k=0;

	scanf("%d-%d-%d",&year,&month,&day);

	if(year<1970||year>9999)
	{
		printf("year is error.\n");
		return 0;
	} 
	else
	{ 
		while(year>i)
		{
			if((i%4==0&&i%100!=0)||i%400==0)
			{
				k++;
			}
			i++;
		}
	}
	n=year-1970;
	x=366*k+365*(n-k);

	if(month>12||month<0)
	{
		printf("month is error.\n");
		return 0;
	}
	else
	{
		if((year%4==0&&year%100!=0)||year%400==0)
		{
			int a1[13]={0,31,29,31,30,31,30,31,31,30,31,30,31};
			for(i=0;i<month;i++)
				sum+=a1[i];
		}
		else
		{
			int a2[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
			for(i=0;i<month;i++)
				sum+=a2[i];
		}
		y=sum;
	}

	if(day<0)
	{
		printf("day is error.\n");
		return 0;
	} 
	else 
	{
		if(month==1||month==3||month==5||month==7||month==8||month==10||month==12)
		{
			if(day>31)
			{
				printf("day is error.\n");
				return 0;
			}
		}
		else if (month==4||month==6||month==9||month==11)
		{
			if(day>30)
			{
				printf("day is error.\n");
				return 0;
			}
		}
		if((year%4==0&&year%100!=0)||year%400==0)
		{
			if(month==2)
			{
				if(day>29) 
				{
					printf("day is error.\n");
					return 0;
				}
			}
		}
		else
		{
			if(month==2)
			{
				if(day>28) 
				{
					printf("day is error.\n");
					return 0;
				}
			}
		}

	}
	z=day;
	p=x+y+z-4;
	week=p%7;
	switch (week)
	{
	case 0: printf("Sunday\n");   break;
	case 1: printf("Monday\n");   break;
	case 2: printf("Tuesday\n");  break;
	case 3: printf("Wednesday\n");break;
	case 4: printf("Thursday\n"); break;
	case 5: printf("Friday\n");   break;
	case 6: printf("Saturday\n"); break;
	}
	return 0;
}