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 
題目含義：給定一個int數，返回其代表的字符串，規則如上

1     public String convertToTitle(int n) {
2          String result = "";
 
3         while (n > 0) {
4             result = (char) (‘A‘ + (n - 1) % 26) + result;
5             n = (n - 1) / 26;
6         }
7         return result;       
8     }

168. Excel Sheet Column Title