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;
}