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

[分析]
26进制转10进制

[注意事项]
1）s == null的处理

[Code]

 1 public class Solution {
 2     public int titleToNumber(String s) 
 3     {
 4         int num =0;
 5         if (s==null) return num;
 6         for (int i =0;i<s.length();i++)
 7         {
 8             num=num*26+(s.charAt(i)-'A'+1);
 9         }
10         
11         return num;
12         
13     }
14 }

reference: http://www.danielbit.com/blog/puzzle/leetcode/leetcode-excel-sheet-column-number