Excel Sheet Column Number

Given a column title as appear in an Excel sheet, return its corresponding column number.

For example:

    A -> 1
    B -> 2
    C -> 3
    ...
    Z -> 26
    AA -> 27
    AB -> 28 

	class Solution {
	public:
		int titleToNumber(string s) 
		{
			int res = 0, mul = 1;
			for (int i = s.size() - 1; i >=0; --i)
			{
				res += (s[i]-'A'+1)*mul;
				mul *= 26;
			}
			return res;
		}
	};