从键盘接收一个字符，如果是小写字符，则转化为相应的大写字符；如果是大写字符，则转化为相应的小写字符；如果是其他字符，则不做转换-Receives a character　the keyboard, if it is lowercase characters, converted to the corresponding uppercase characters　if uppercase characters are converted to the corresponding lowercase characters　if other characters are not converted