Project Euler Problem 19 Counting Sundays

Counting Sundays

Problem 19

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)?

C++:

#include <iostream>

using namespace std;

const int DAYS = 7;
enum DAY {SUN=0, MON, TUE, WED, THU, FRI, SAT};
int days[]={31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

// 闰年计算函数
int leapyear(int year) {
if((year%4 == 0 && year%100 != 0) || year%400 == 0)
return 1;
return 0;
}

int main()
{
int sum=0, startday = MON;

for(int i=1900; i<=2000; i++) {
days[1] = leapyear(i) ? 29 : 28;
for(int j=0; j<12; j++) {
startday += days[j];
startday %= DAYS;
if(startday == SUN) // first is SUN day
if(i >= 1901)
sum++;
}
}
if(startday == SUN)
sum--;

cout << sum << endl;

return 0;
}