Project Euler 19: Counting Sundays

You are given the following information, but you may prefer to do some research for yourself.

• 1 Jan 1900 was a Monday.
• Thirty days has September,
  April, June and November.
  All the rest have thirty-one,
  Saving February alone,
  Which has twenty-eight, rain or shine.
  And on leap years, twenty-nine.
• A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

此题要求计算从1901年01月01日到2000年12月31日期间，有多少周日落在每月一日。

开两个数组分别存放平、闰年每月天数（当然只有2月份不同），再开一个1200的数组存放每月一日是第几天（1901年01月01日算作第一天）。由已知推算1901年01月01日为周三，那么显然在1200的数组中对模7同余5的便是周日。

代码如下：

#include<stdio.h>int main(void){        int a[12]={31,28,31,30,31,30,31,31,30,31,30,31};    int b[12], c[1200];    int i, y, n=0, count=0;        for(i=0;i<12;i++)        b[i]=a[i];    b[1]=29;    c[0]=1;    for(y=1901; y<=2000; y++){        if(y%4!=0)            for(i=0; i<12; i++)                c[i+12*n+1]=c[i+12*n]+a[i];        else            for(i=0; i<12; i++)                c[i+12*n+1]=c[i+12*n]+b[i];        n++;    }    for(i=0; i<1200; i++)        if(c[i]%7==5)            count++;    printf("%d\n",count);        return 0;}

最终结果为171.