不调用库函数同时实现strlwr()与strupr()

#include
#include
main()
{
    char string[10];
    int i = 0;
    gets(string);
    while(string[i] != '\0')
    {
       if(string[i] >= 'A' && string[i] <= 'Z')
            string[i] += 32;
       else if(string[i] >= 'a' && string[i] <= 'z')
            string[i] -= 32;
       i++;
    }
    puts(string);
}