[leetcode]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 

[Thought]

The essence of this problem is the number system , like change the twenty-six number system into decimal system . 

Code:

public class Solution {
    public static int titleToNumber(String s) {
        if( s == null)
            return 0;
        int len = s.length();
        int total = 0;
        for(int i=0;i<len;i++){
            char x = s.charAt(i);
            total += Math.pow(26, len-1-i)*(x-64);
        }
        return total;
    }
}