504. 七进制数

class Solution {
    public String convertToBase7(int num) {
        if (num == 0)
        {
            return "0";
        }
        boolean isNegative = num < 0;
        StringBuilder sb = new StringBuilder();
        if(isNegative)
        {
            num = 0 - num;
        }
        while(num != 0)
        {
            sb.append(num % 7);
            num = num / 7;
        }
        String ret = sb.reverse().toString();
        if (isNegative)
        {
            return "-" + ret;
        }
        return ret;
    }
}