帮帮我解一下这道题吧

Question

Answer 1

#include<stdio.h>  
#include<math.h>  
int main()  
{     
    int a,b1,b2,b3 ;
    scanf("%d",&a);
    if(a<100||a>1000) 
        return 0;
    b1=a/100;
    b2=a%100/10;
    b3=a%10;
    if(b3==0&&b2==0)
        printf("%d",b1);
    else if(b3==0)
        printf("%d%d",b2,b1);
    else 
        printf("%d%d%d",b3,b2,b1);
    return 0;  
}