HDU 2133 What day is it

http://acm.hdu.edu.cn/showproblem.php?pid=2133

Problem Description

Today is Saturday, 17th Nov,2007. Now, if i tell you a date, can you tell me what day it is ?

 

Input

There are multiply cases.
One line is one case.
There are three integers, year(0<year<10000), month(0<=month<13), day(0<=day<32).

 

Output

Output one line.
if the date is illegal, you should output "illegal". Or, you should output what day it is.

 

Sample Input

2007 11 17

 

Sample Output

Saturday

 

时间复杂度：$O(1)$

代码：

#include <bits/stdc++.h>
using namespace std;

int a[13] = {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int b[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
char s[8][10] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
bool IsRunNian(int year) {
    if((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
        return true;
    return false;
}
int main() {
    int year, month, day;
    while(~scanf("%d%d%d", &year, &month, &day)) {
        if(IsRunNian(year)) {
            if(day > a[month] || month == 0 || day == 0) {
                printf("illegal\n");
                continue;
            }
        } else{
            if(day > b[month] || month == 0 || day == 0) {
                printf("illegal\n");
                continue;
            }
        }
        int sum = 0;
        for(int i = 1; i < year; i ++) {
            if(IsRunNian(i))
                sum += 366;
            else
                sum += 365;
            sum %= 7;
        }
        for(int i = 0; i < month; i ++) {
            if(IsRunNian(year))
                sum += a[i];
            else
                sum += b[i];
            sum %= 7;
        }

        sum += day;
        sum %= 7;
        printf("%s\n",s[sum]);
    }
    return 0;
}