Excel Sheet Column Number

Problem:

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 

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