Base 7
Given an integer num, return a string of its base 7 representation.
Example 1:
Input: num = 100
Output: "202"
Example 2:
Input: num = -7
Output: "-10"
Constraints:
-107 <= num <= 107
思路一: 辗转相除法，负数先转成正数再计算
public String convertToBase7(int num) {
if (num == 0) return "0";
StringBuilder result = new StringBuilder();
boolean negative = num < 0;
num = Math.abs(num);
while (num != 0) {
result.append(num % 7);
num /= 7;
}
return negative ?
"-" + result.reverse() :
result.reverse().toString();
}