string 轉換成大寫（或小寫）

char exchange(char c)
{
    if (c <= 'Z' && c >= 'A')
        c = tolower(c);

    else if (c >= 'a' && c <= 'z')
        c = toupper(c);

    return c;
}

int main()
{
    string strTmp = "short";
	transform(strTmp.begin(), strTmp.end(), strTmp.begin(), ::toupper); //轉換成大寫
	transform(strTmp.begin(), strTmp.end(), strTmp.begin(), ::tolower); //轉換成小寫
	transform(strTmp.begin(), strTmp.end(), strTmp.begin(), exchange);  //大小寫互換
}