HDU 2133 What day is it（给定日期求星期几）

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本文介绍了一个用于将给定日期转换为对应的周几的算法实现过程，包括闰年的判断、月份天数的计算以及日期转换逻辑，通过代码展示了具体实现方法。

Problem Description

Today is Saturday, 17th Nov,2007. Now, if i tell you a date, can you tell me what day it is ?

Input

There are multiply cases.
One line is one case.
There are three integers, year(0<year<10000), month(0<=month<13), day(0<=day<32).

Output

Output one line.
if the date is illegal, you should output "illegal". Or, you should output what day it is.

Sample Input

2007 11 17

Sample Output

Saturday

思路见AC代码：
#include<stdio.h>
int run(int m)
{
    if((m%4==0&&m%100!=0)||(m%400==0))
        return 1;
    else
        return 0;
}
void print(int m)
{
    switch(m)
    {
    case 2:
        printf("Monday\n");
        break;
    case 3:
        printf("Tuesday\n");
        break;
    case 4:
        printf("Wednesday\n");
        break;
    case 5:
        printf("Thursday\n");
        break;
    case 6:
        printf("Friday\n");
        break;
    case 0:
        printf("Saturday\n");
        break;
    case 7:
         printf("Saturday\n");
        break;
    case 1:
        printf("Sunday\n");
        break;
    }
}
int main()
{
    int a[]= {0,31,28,31,30,31,30,31,31,30,31,30,31};
    int dup,dn;
    int y,m,d;
    int sd,i;                             //天数总差
    int weak,x;
    dn=0;
    for(i=1; i<11; i++)
        dn=dn+a[i];
    dn=dn+17;
    dup=0;
    dup=dup+a[12]+13;
    int ffff;
    while(scanf("%d%d%d",&y,&m,&d)!=EOF)
    {
        ffff=0;
        if(0<y&&y<10000&&0<m&&m<13&&0<d&&d<32)  //细节很重要，这里WA好几次....
        {
            if(d<=a[m]||(m==2&&d<=29&&run(y)))
            {
                ffff=1;
            }
        }
        sd=0;
        if(ffff)
        {
            if(y>2007)
            {
                x=y-2007-1;
                sd=sd+x*365;
                for(i=2007; i<y; i++)
                {
                    if(run(i))
                        sd++;
                }
                for(i=1; i<m; i++)
                    sd=sd+a[i];             //加上月
                sd=sd+d;                //加上天
                if(m>2&&run(y))         //如果本年是闰年且月份大于2
                    sd++;
                sd=sd+dup;
                weak=(sd)%7;            //一周7天,因为2007,11,17星期六,所以sd+1
                print(weak);
            }
            else if((y==2007&&m>11)||(y==2007&&m==11&&d>=17))
            {
                if(m>11)
                    sd=sd+13+d;
                else
                    sd=sd+d-17;
                weak=(sd)%7;            //一周7天,因为2007,11,17星期六,所以sd+1
                print(weak);
            }
            else if(y<2007)
            {
                x=2007-(y+1);
                sd=sd+x*365;
                for(i=y+1; i<2007; i++)
                {
                    if(run(i))
                        sd++;
                }
                for(i=m+1; i<=12; i++)
                    sd=sd+a[i];
                if(m<=2&&run(y))
                    sd++;
                sd=sd+(a[m]-d);
                sd=sd+dn;
                weak=7-(sd)%7;            //一周7天,因为2007,11,17星期六,所以sd+1
                print(weak);
            }
            else if((y==2007&&m<11)||(y==2007&&m==11&&d<17))
            {
                if(m<11)
                {
                    for(i=m+1; i<11; i++)
                        sd=sd+a[i];
                    sd=sd+(a[m]-d)+17;
                }
                else
                {
                    sd=sd+(17-d);
                }
                weak=7-(sd)%7;            //一周7天,因为2007,11,17星期六,所以sd+1
                print(weak);
            }
        }
        else
        {
            printf("illegal\n");
        }
    }
    return 0;
}

再上一个公式：
int whatday(int y,int m,int d)  //蔡勒公式.注意：只适用1582-10-5之后
{
    int ans ;
    if(m==1||m==2)
    m=m+12,y--;
    if((y<1752)||(y==1752&&m<9)||(y==1752&&m==9&&d<3))
    ans=(d+2*m+3*(m+1)/5+y+y/4+5)%7;             //1752-9-3之前 
    else
    ans=(d+2*m+3*(m+1)/5+y+y/4-y/100+y/400)%7;   //1752-9-3之后（包括9-3）
    return ans;                                  //返回值+1就是对应星期几
}