Leetcode 171. Excel Sheet Column Number

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

For example:

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

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