Excel Sheet Column Number

https://www.lintcode.com/problem/excel-sheet-column-number/description

public class Solution {
    /**
     * @param s: a string
     * @return: return a integer
     */
    public int titleToNumber(String s) {
        // write your code here
        int res = 0;
        char[] chars = s.toCharArray();
        int factor = 1;
        for (int i = chars.length - 1; i >= 0; i--) {
            char aChar = chars[i];
            int index = aChar - 'A' + 1;
            res += index * factor;
            factor *= 26;
        }
        return res;
    }
}