LeetCode: 171 Excel Sheet Column Number(easy)

题目：

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

代码：

class Solution {
public:
    int titleToNumber(string s) {
        int num = 0;
        for (int i = 0; i < s.length(); i++)
        {
            char ch = s[i];
            num = num * 26 + (ch - 'A' + 1);
        }
        
        return num;
    }
};