Problem 19 of How many Sundays fell on the first of the month during the twentieth century

http://projecteuler.net/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)?

• 1900年1月1日是星期一。
• 30天的月份有：9月，4月，6月，11月。
• 此外的月份都是31天，當然2月除外。
• 2月在閏年有29天，其他時候有28天。
• 年份可以被4整除的時候是閏年，但是不能被400整除的世紀年（100的整數倍年）除外。

20世紀（1901年1月1日到2000年12月31日）一共有多少個星期日落在了當月的第一天？

public class TwentiethCentury {
	
	public static void main(String args[]){
		int all=0;
		for(int i=1901;i<=2000;i++){
			all+=everyMonth(i, i);
		}
//		System.out.println(everyMonth(1901, 1901));
		System.out.println(all);
	}
	
	public static boolean isLeap(int year){
		if((year%4==0&&year%100!=0)||(year%400==0))
			return true;
		return false;
	}
	
	public static int everyMonth(int year,int leave){
		int sum=0;
		int j=0;
		int num=leaveNum(leave);
		for(int i=1;i<=12;i++){
			if(i==1){
				sum=num+1;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==2){
				sum+=31;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==3){
				if(isLeap(year)){
					sum+=29;
					if(isSunday(sum)){
						j++;
						System.out.println("year="+year+",month="+i+",sum="+sum);
					}
				}else{
					sum+=28;
					if(isSunday(sum)){
						j++;
						System.out.println("year="+year+",month="+i+",sum="+sum);
					}
				}
			}else if(i==4){
				sum+=31;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==5){
				sum+=30;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==6){
				sum+=31;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==7){
				sum+=30;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==8){
				sum+=31;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==9){
				sum+=31;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==10){
				sum+=30;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==11){
				sum+=31;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}else if(i==12){
				sum+=30;
				if(isSunday(sum)){
					j++;
					System.out.println("year="+year+",month="+i+",sum="+sum);
				}
			}
		}
		return j;
	}
	
	public static boolean isSunday(int num){
		if(num%7==0)
			return true;
		return false;
	}
	
	public static int leaveNum(int year){
		int sum=0;
		if(year==1901){
			sum=0;
			System.out.println("leaveNum="+sum);
		}else if((year-1901)%4==0){
			sum+=(year-1901)/4*(365*3+366);
			System.out.println("leaveNum="+sum);
		}else if((year-1901)%4==1){
			sum+=(year-1901)/4*(365*3+366)+365;
			System.out.println("leaveNum="+sum);
		}else if((year-1901)%4==2){
			sum+=(year-1901)/4*(365*3+366)+365*2;
			System.out.println("leaveNum="+sum);
		}else if((year-1901)%4==3){
			sum+=(year-1901)/4*(365*3+366)+365*2+366;
			System.out.println("leaveNum="+sum);
		}
		return sum%7;
	}
}

Answer:

171