用C++编写一个具有如下功能的万年历与日历查询系统：显示某一年的全年日历（每天要有对应的星期）；显示任意指定某年某月的月历（每天要有对应的星期）；显示某年某月某日是星期几；显示某年某月某日是该年的第几天。已知1980年1月1日是星期二。

思路：

根据1980年1月1日是星期二，我们可以求出该年每个月的第一天是星期几，以及该年的每一天是星期几。
实现时，我们可以定义一个二维数组，第一维表示月份，第二维表示天数。对于某一天，我们可以通过算出它距离1980年1月1日有多少天，然后对7取模，得到该天是星期几。

代码如下：

#include <stdio.h>

// 每个月有多少天
int daysOfMon[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

// 1980年1月1日是星期二
int base = 2;

// 判断是否为闰年
int isLeap(int year) {
    if((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
        return 1;
    } else {
        return 0;
    }
}

// 算出某一天是星期几
int getWeekday(int year, int month, int day) {
    int i, days = 0;
    for(i = 1980; i < year; i++) {
        if(isLeap(i)) {
            days += 366;
        } else {
            days += 365;
        }
    }

    for(i = 1; i < month; i++) {
        days += daysOfMon[i];
        if(i == 2 && isLeap(year)) {
            days++;
        }
    }

    days += day - 1;

    return (base + days) % 7;
}

// 显示某一年的全年日历
void showYear(int year) {
    printf("================= %d ====================\n", year);
    printf("   J     F     M     A     M     J     J     A     S     O     N     D\n");

    int i, j, month, days;
    for(month = 1; month <= 12; month++) {
        days = daysOfMon[month];
        if(month == 2 && isLeap(year))
            days++;
        printf(" ");
        for(i = 0; i < month-1; i++) {
            printf("      ");
        }

        for(i = 1; i <= days; i++) {
            printf("%2d", i);
            int day = getWeekday(year, month, i);
            if(day == 0) {
                printf(" Su  ");
            } else if(day == 1) {
                printf(" Mo  ");
            } else if(day == 2) {
                printf(" Tu  ");
            } else if(day == 3) {
                printf(" We  ");
            } else if(day == 4) {
                printf(" Th  ");
            } else if(day == 5) {
                printf(" Fr  ");
            } else if(day == 6) {
                printf(" Sa  ");
            }
        }
        printf("\n");
    }
}

// 显示任意指定某年某月的月历
void showMonth(int year, int month) {
    printf("=============== %d.%d ===============\n", year, month);
    printf(" Su  Mo  Tu  We  Th  Fr  Sa\n");
    int i, j, days = daysOfMon[month];
    if(month == 2 && isLeap(year))
        days++;

    for(i = 1; i <= days; i++) {
        int day = getWeekday(year, month, i);
        if(i == 1) {
            for(j = 0; j < day; j++) {
                printf("    ");
            }
        }
        printf("%2d", i);
        if(day == 0) {
            printf(" Su  ");
        } else if(day == 1) {
            printf(" Mo  ");
        } else if(day == 2) {
            printf(" Tu  ");
        } else if(day == 3) {
            printf(" We  ");
        } else if(day == 4) {
            printf(" Th  ");
        } else if(day == 5) {
            printf(" Fr  ");
        } else if(day == 6) {
            printf(" Sa  ");
        }
        if((day+i) % 7 == 0) {
            printf("\n");
        }
    }
    printf("\n");
}

// 显示某年某月某日是星期几
void showWeekday(int year, int month, int day) {
    int weekday = getWeekday(year, month, day);
    printf("%d.%d.%d is ", year, month, day);
    if(weekday == 0) {
        printf("Sunday\n");
    } else if(weekday == 1) {
        printf("Monday\n");
    } else if(weekday == 2) {
        printf("Tuesday\n");
    } else if(weekday == 3) {
        printf("Wednesday\n");
    } else if(weekday == 4) {
        printf("Thursday\n");
    } else if(weekday == 5) {
        printf("Friday\n");
    } else if(weekday == 6) {
        printf("Saturday\n");
    }
}

// 显示某年某月某日是该年的第几天
void showDay(int year, int month, int day) {
    int i, days = 0;
    for(i = 1; i < month; i++) {
        days += daysOfMon[i];
        if(i == 2 && isLeap(year)) {
            days++;
        }
    }

    days += day;

    printf("%d.%d.%d is the %dth day of %d\n", year, month, day, days, year);
}

// main函数
int main() {
    int year, month, day;
    printf("Please enter year(YYYY): ");
    scanf("%d", &year);
    printf("Please enter month(MM): ");
    scanf("%d", &month);
    printf("Please enter day(DD): ");
    scanf("%d", &day);

    showYear(year);
    showMonth(year, month);
    showWeekday(year, month, day);
    showDay(year, month, day);

    return 0;
}