C語言：編程求任意月份的天數

閏年問題，因為二月份的天數與閏年有關。閏年的判斷依據是：若某年能被4整除，但不能被100整除，則這一年是閏年；若某年能被400整除，則這一年也是閏年

#include <stdio.h>
#include <stdlib.h>
main(){ 
    int year, month, days;
    while (1)
    {
    
        printf("輸入年份：");
        scanf("%d",&year);
        if (year==9999)
            break;
        printf("輸入月份："); 
        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:
                // 判斷閏年
                if(year%4==0 && year%100!=0 || year%400==0)
                    days=29; 
                else
                    days=28; 
                break; 
            default:
                printf("月份輸入錯誤！\n");
                exit(1);
                break; 
        } 
        printf("天數：%d\n", days); 
    }
}

 改進1：bz為1時表示月份輸入錯誤，不用輸出天數

#include <stdio.h>
#include <stdlib.h>
main(){ 
    int year, month, days,bz;
    while (1)
    {
        bz=0;
        printf("輸入年份：");
        scanf("%d",&year);
        if (year==9999)
            //break;
            exit(1);
        printf("輸入月份："); 
        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:
                // 判斷閏年
                if(year%4==0 && year%100!=0 || year%400==0)
                    days=29; 
                else
                    days=28; 
                break; 
            default:
                printf("月份輸入錯誤！\n");
                bz=1;
                //exit(1);
                break; 
        } 
        if (bz==0)
               printf("天數：%d\n", days);  
    }
}