C language realizes inputting a number less than 1000 from the keyboard, and judging whether it is a self-preserved number

Enter a natural number less than 1000 from the keyboard to determine whether it is an autologous number. An autologous number means that the mantissa of the square of a number is equal to its own natural number. For example, 25*25=625
is defined by the autologous number. The remainder of the square of a number divided by 10 or 100 or 1000 is the automorphic number itself. The
code is as follows:

#include <stdio.h>

void main(){
    
    
	int num = 0,div=0;
	printf("请输入自然数\n");
	scanf("%d",&num);
	if(num<10){
    
    
		div = 10;
	}else if(num < 100){
    
    
		div = 100;
	}else{
    
    
		div =1000;
	}
	if(num * num % div == num){
    
    
		printf("%d 是自守数\n",num);
	}else{
    
    
		printf("%d 不是自守数\n",num);
	}

}