judge4-2.c

#include <stdio.h>
//return if input is leap year
int leap(int year) {
    return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
}
//return leap day between base day and target day.
int leapDay(int year, int month, int date) {
    int count = 0, i;
    //大於等於基準日 2000 / 01 / 01
    if(year >= 2000) {
        for(i = 2000; i <= year; i++) {
            if(leap(i)) count ++;
        }
        if(leap(year) && month < 3)
            count --;
    //小於基準日 2000 / 01 /01
    } else {
        for(i = year; i < 2000; i++) {
            if(leap(i)) count ++;
        }
        if(leap(year) && (month > 3))
            count --;
        count *= -1; // 小於基準日，閏日改為負數
    }
    return count;
}

int main() {
    int year, month, date, day;
    //store month code
    int monthCode[12] = {5, 1, 1, 4, 6, 2, 4, 0, 3, 5, 1, 3};
    scanf("%d %d %d", &year, &month, &date);
    day = (year - 2000 + leapDay(year, month, date) + monthCode[month-1] + date) % 7;
    printf("%d\n", day >= 0 ? day : day + 7);
    return 0;
}