504. Base 7

Given an integer, return its base 7 string representation.

Example 1:

Input: 100
Output: "202"

Example 2:

Input: -7
Output: "-10"

Note: The input will be in range of [-1e7, 1e7].

public class Solution {
    public String convertToBase7(int num) {
        StringBuilder sb = new StringBuilder();
        StringBuilder r = new StringBuilder();
        if (num == 0) return "0";
        if (num < 0) {
            r.append('-');
            num = -num;
        }
        while (num != 0) {
            sb.append((char)(num % 7 + '0'));
            num /= 7;
        }        
        return