C语言：编程求任意月份的天数

闰年问题，因为二月份的天数与闰年有关。闰年的判断依据是：若某年能被4整除，但不能被100整除，则这一年是闰年；若某年能被400整除，则这一年也是闰年

#include <stdio.h>
#include <stdlib.h>
main(){ 
    int year, month, days;
    while (1)
    {
    
        printf("输入年份：");
        scanf("%d",&year);
        if (year==9999)
            break;
        printf("输入月份："); 
        scanf("%d", &month);
        switch(month){
            case 1: 
            case 3: 
            case 5: 
            case 7: 
            case 8: 
            case 10: 
            case 12:
                days=31;
                break; 
            case 4: 
            case 6: 
            case 9: 
            case 11:
                days=30;
                break; 
            case 2:
                // 判断闰年
                if(year%4==0 && year%100!=0 || year%400==0)
                    days=29; 
                else
                    days=28; 
                break; 
            default:
                printf("月份输入错误！\n");
                exit(1);
                break; 
        } 
        printf("天数：%d\n", days); 
    }
}

 改进1：bz为1时表示月份输入错误，不用输出天数

#include <stdio.h>
#include <stdlib.h>
main(){ 
    int year, month, days,bz;
    while (1)
    {
        bz=0;
        printf("输入年份：");
        scanf("%d",&year);
        if (year==9999)
            //break;
            exit(1);
        printf("输入月份："); 
        scanf("%d", &month);
        switch(month){
            case 1: 
            case 3: 
            case 5: 
            case 7: 
            case 8: 
            case 10: 
            case 12:
                days=31;
                break; 
            case 4: 
            case 6: 
            case 9: 
            case 11:
                days=30;
                break; 
            case 2:
                // 判断闰年
                if(year%4==0 && year%100!=0 || year%400==0)
                    days=29; 
                else
                    days=28; 
                break; 
            default:
                printf("月份输入错误！\n");
                bz=1;
                //exit(1);
                break; 
        } 
        if (bz==0)
               printf("天数：%d\n", days);  
    }
}