欧拉计划Problem 19

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本文探讨了如何计算从1901年到2000年间，每个月的第一天为周日的次数。通过编程实现，包括判断闰年的规则和每月天数的调整。

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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)?

int main()
{
clock_t te,ts;
ts=clock();
int month[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
int day = 2,count=0;
for (int year = 1901; year <= 2000; year++)
{
  if (year % 100 == 0)
  {
   if (year % 400 == 0)
   {
    month[2]++;
   }
  }
  else
  {
   if (year % 4 == 0)
   {
    month[2]++;
   }
  }
  for (int j =1; j <= 12; j++)
  {
   day = (day+month[j]% 28)% 7;
   if (day == 0)
   {
    count++;
   }
  }
  month[2] = 28;
}
printf("answer: %d",count);
te=clock();
printf("\ntime difference: %ds\n",(te-ts)/CLOCKS_PER_SEC);
return 0;
}