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

这道题是Excel Sheet Column Number 的反算，关键就是利用mod运算算出最后一位的字母，然后需要(n-1)/26来算出下一个数字, 需要n-1是因为从27开始会变成2位数。

public String convertToTitle(int n) {
        List<String> letters = Arrays.asList(" ", "A" ,"B", "C", "D", "E", "F", "G", "H", "I", "J"
                , "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T",
                  "U", "V", "W", "X", "Y", "Z");

        StringBuilder sb = new StringBuilder();
        while (n!=0) {
            int number = n % 26;
            if (number==0) {
                sb.insert(0, letters.get(26));
            } else {
                sb.insert(0, letters.get(number));
            }
            n = (n-1)/26;
        }
        return sb.toString();
    }