(Problem 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)?

 

#include <stdio.h> 

#include <stdbool.h>



const int a[2][12] = {{31,28,31,30,31,30,31,31,30,31,30,31},

                     {31,29,31,30,31,30,31,31,30,31,30,31}};





bool leapYear(int n)  //判断闰年

{

    return (((n % 4 ==0) && (n % 100 !=0)) || (n % 400 == 0));

}



bool issunday(int n) //判断某天是否是星期天

{

    return (n % 7 == 0 ? true : false);

}



void solve(void)

{

    int num, i, j, count;

    count = 0;



    i = 1901;

    num = 1;

    while(i < 2000) {



        int t = (leapYear(i) ? 1 : 0);   //判断闰年

        for(j = 0; j < 12; j++) {

            num += a[t][j];

            if(issunday(num)) count++;

        }

        i++;

    }

    printf("%d\n",count);

}



int main(void)

{

    solve();

    return 0;

}

Answer:

171