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 
    ...

Example 1:

Input: 1
Output: "A"

Example 2:

Input: 28
Output: "AB"

Example 3:

Input: 701
Output: "ZY"

convert base-10 number system to base-26 number system

time = O(n ^ 2), space = O(n) -- extra space O(n) for the answer

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