思想：

使用二分的方法判斷

package cn.liyi.day03;

import java.util.Scanner;

public class Demo190 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double x = sc.nextInt();
        double l = -10000, r = 10000;
        while(r - l > 1e-8) {
            double mid = (l + r) / 2;
            if(mid * mid * mid >= x)
                r = mid;
            else
                l = mid;
        }
        System.out.printf("%.6f\n",l);
//        System.out.println(l);
    }

}