171. Excel Sheet Column Number

Given a string columnTitle that represents the 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 columnTitle) {
        int res = 0;
        for(auto ch : columnTitle) {
            res = res * 26 + (ch - 'A' + 1);
        }
        return res;
    }
};