504. Base 7

public class Solution {
    public String convertToBase7(int num) {
        int negative = 1;
        if(num < 0){
            negative = -1;
            num = - num;
        }
        StringBuffer sb = new StringBuffer();
        while(num > 6){
            sb.append(num % 7);
            num = num / 7;
        }
        sb.append(num);
        if(negative < 0){
            sb.append("-");
        }
        return sb.reverse().toString();
    }
}