22、输入月份输出天数，输入一个数字n(1~12),其中2月为28天 23、输出n的阶乘 输入一个数n 输出n的阶乘C语言

在C语言中，你可以分别编写两个函数来解决这两个问题：

1. **输入月份输出天数**:

#include <stdio.h>

int days_in_month(int month) {
    switch(month) {
        case 1: // January
        case 3: // March
        case 5: // May
        case 7: // July
        case 8: // August
        case 10: // October
        case 12: // December
            return 31;
        case 4: // April
        case 6: // June
        case 9: // September
        case 11: // November
            return 30;
        case 2: // February (accounting for leap year)
        default:
            if ((month == 2 && calendar_is_leap_year()) || month != 2) {
                return 28; // Standard years have 28 days in February
            } else {
                return 29; // Leap years have 29 days in February
            }
    }
}

// Function to check if a year is a leap year (using the common rule)
int calendar_is_leap_year() {
    int year;
    scanf("%d", &year);
    if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) {
        return 1;
    }
    return 0;
}

int main() {
    int n;
    printf("Enter a month between 1 and 12: ");
    scanf("%d", &n);
    printf("Days in %dth month: %d\n", n, days_in_month(n));
    return 0;
}

这个程序首先询问用户输入月份，然后根据给定的月份返回相应的天数，会考虑闰年的情况。

2. **输出n的阶乘**:

#include <stdio.h>

long long factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

int main() {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    printf("The factorial of %d is: %lld\n", n, factorial(n));
    return 0;
}

这个函数递归地计算给定数字的阶乘，并将结果作为长整型返回，因为较大的阶乘值可能会超出普通整数范围。