168. Excel Sheet Column Title

Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

    1 -> A
    2 -> B
    3 -> C
    ...
    26 -> Z
    27 -> AA
    28 -> AB 

做的时候要先把AB=》数字的公式写出来，然后根据那个公式搞
因为里边A相当于1的角色，并且A最小。
十进制里边0最小 

//AAZ -> 1*26^2+1*26^1+26
//AAB -> 1*26^2+1*26^1+2

 public class Solution {  
   public String convertToTitle(int n) {  
     StringBuilder sb = new StringBuilder();  
     while(n>0){  
       int r = (n-1)%26;  
       n = (n-1)/ 26;  
       sb.insert(0,(char)('A'+r));  
     }  
     return sb.toString();  
   }  
 }