Leetcode: 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 

Credits:
Special thanks to @ifanchu for adding this problem and creating all test cases.

Understand the problem:
A classic 10-based math to 26-based math. Remember for a 10-based integer, e.g. 123, how can we extract each digit?  The idea is from the least significant digit and use %10 and /10 respectively. e.g. 
123 % 10 = 3, 123 / 10 = 12
12 % 10 = 2, 12 / 10 = 1
1 % 10 = 1, 1 / 10 - 0.

Therefore, for this question, we only need to replace 10 to 26. Note that for this question, the map is from 1 to 26. Therefore, we need to minus 1 for each number before the computation.  

Code (Java):

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public class Solution {     public String convertToTitle(int n) {         if (n <= 0) {             return "";         }                   StringBuffer sb = new StringBuffer();         convertToTitleHelper(n, sb);                   sb.reverse();                   return sb.toString();     }           private void convertToTitleHelper(int n, StringBuffer sb) {         if (n <= 0) {             return;         }         n--;         int val = n % 26;         val = val < 0 ? val + 26 : val;         char title = (char) (val + 'A');                   sb.append(title);                   convertToTitleHelper(n / 26, sb);     } }

Update on 10/14/15:

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public class Solution {     public String convertToTitle(int n) {         if (n <= 0) {             return "";         }                   StringBuffer sb = new StringBuffer();                   while (n > 0) {             n--;                           int val = n % 26;             char title = (char) (val + 'A');             sb.insert(0, title);                           n /= 26;         }                   return sb.toString();     } }