计算浮点数的立方根

二分查找
注意浮点数如果为负数的情况，因为负数也可以开立方根

  public static double cubeRoot(double num) {
        double left = 0;
        double right = num;

        if (num < 0) {
            left = num;
            right = 0;
        }

        double mid = (left + right) / 2;
        double ans = mid* mid *mid;
        while (Math.abs(num - (ans)) > 1e-5) {
            mid = (left + right) / 2;
            ans = mid * mid * mid;
            if (ans - num > 1e-5) {
                right = mid;
            } else if (num - ans > 1e-5){
                left = mid;
            }
        }

        return mid;
    }