[leetcode] problem 504 – base 7

public String (int num) {
    if (num == 0)
        return "0";
    
    StringBuilder sb = new StringBuilder();
    boolean positive = num >= 0;
    num = Math.abs(num);

    while (num > 0) {
        sb.append(num % 7);
        num /= 7;
    }

    String result = sb.reverse().toString();
    return positive ? result : "-" + result;
}