1.根据二进制规律很容易就发现

char toLower(char x)
{if (x >= 'A' && x <= 'Z')return  (x | 0x20);return x;
}char    toUpper(char x)
{if (x >= 'a' && x <= 'z')return  (x  & 0xDF);return x;
}

2. 根据ASCII 规律

char toLower(char x)
{if (x >= 'A' && x <= 'Z')return  (x+ 'a'-'A');return x;
}char toUpper(char x)
{if (x >= 'a' && x <= 'z')return  (x - 'a' + 'A');return x;
}