题目：

题解：

class Solution {public String convertToBase7(int num) {if (num == 0) {return "0";}boolean negative = num < 0;num = Math.abs(num);StringBuffer digits = new StringBuffer();while (num > 0) {digits.append(num % 7);num /= 7;}if (negative) {digits.append('-');}return digits.reverse().toString();}
}