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):

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:

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();
    }
}