171. Excel Sheet Column Number

题目来源【Leetcode】

Related to question Excel Sheet Column Title

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

这道题就是纯数学的题：

class Solution {public:    int titleToNumber(string s) {        int sum = 0;        reverse(s.begin(),s.end());        for(int i = 0; i < s.length(); i++){            sum += (s[i]-'A'+1)*pow(26,i);        }        return sum;    }};