leetcode--Excel Sheet Column Number

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 

public class Solution {
    public int titleToNumber(String s) {
        String letters="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        int length = s.length();
        int sum=0;
        for(int i = 0; i < length; i++)
           sum+=Math.pow(26,length-i-1)*(letters.indexOf(s.charAt(i))+1);
        return sum;
        }
        
    }