Leetcode 171 Excel Sheet Column Number

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

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