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

**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(); } }

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