```
public class Solution {
public String convertToBase7(int num) {
if(num == 0) return "0";
boolean isNagative = true;
if(num >= 0) isNagative = false;
num = Math.abs(num);
int [] base7 = {1, 7, 49, 343, 2401, 16807, 117649, 823543, 5764801, 40353607, 282475249};
StringBuilder result = new StringBuilder(11);
int i = 10;
for(; i >= 0; i--) {
int digit = num / base7[i];
result.append(digit);
num -= digit * base7[i];
}
while(result.charAt(++i) == '0');
return isNagative ? "-" + result.substring(i) : result.substring(i);
}
}
```