168. Excel表列名称

class Solution {
    public String convertToTitle(int columnNumber) {
        int num = columnNumber;
        StringBuffer sb = new StringBuffer();
        while (num != 0) {
            num--; // 注意
            sb.append((char)(num % 26 + (int) 'A'));
            num /= 26;
        }
        return sb.reverse().toString();
    }
}