Solution to Project Euler 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)?

• Java Code

package sikademy;
/**
 *
 * @author Archangel Macsika
 * Copyright (c) Sikademy. All rights reserved
 */

public class SikademyEulerSolution {
	
	public String run() {
		int count = 0;
		for (int y = 1901; y <= 2000; y++) {
			for (int m = 1; m <= 12; m++) {
				if (dayOfWeek(y, m, 1) == 0)  // Sunday
					count++;
			}
		}
		return Integer.toString(count);
	}
	
	
	// Return value: 0 = Sunday, 1 = Monday, ..., 6 = Saturday.
	private static int dayOfWeek(int year, int month, int day) {
		if (year < 0 || year > 10000 || month < 1 || month > 12 || day < 1 || day > 31)
			throw new IllegalArgumentException();
		
		// Zeller's congruence algorithm
		int m = (month - 3 + 4800) % 4800;
		int y = (year + m / 12) % 400;
		m %= 12;
		return (y + y/4 - y/100 + (13 * m + 2) / 5 + day + 2) % 7;
	}

	public static void main(String[] args) {
                SikademyEulerSolution solution = new SikademyEulerSolution();
		System.out.println(solution.run());
	}	
}

• Python Code

# 
# @author Archangel Macsika
# Copyright (c) Sikademy. All rights reserved.
# 

import datetime

def compute():
	ans = sum(1
		for y in range(1901, 2001)
		for m in range(1, 13)
		if datetime.date(y, m, 1).weekday() == 6)
	return str(ans)


if __name__ == "__main__":
	print(compute())