Excel Sheet Column Title.cpp finished

for normal numbers, such as 13, 21, 27, 36, we can use this algorithm
	while(n > 26) {
		ret = n % 26;
		n /= 26;
		result += dic[ret];
	}

however, when it comes with carry numbers, such as 52 = 26 * 2;

the right result is AZ, but our program will give B0

so, when ret == 0, we should give dic[ret] = 'Z', and Decrease n--