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 
     
      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;}