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 

从左到右，类似于26进制

public class Solution {
    public int titleToNumber(String s) {
        int res = 0;
        for(int i=0; i<s.length(); i++){
            char ch = s.charAt(i);
            res = res*26 + ch - 'A' + 1;
        }
        return res;
    }
}