LeetCode[Math]: Excel Sheet Column Number

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

这个题目很简单，我的C++代码实现如下：

int titleToNumber(string s) {
    int res = 0;
    for (auto c : s)
        res = res * 26 + (c - 'A' + 1);

    return res;
}