AC呆马：

#include <iostream>
#include <cstdio>
using namespace std;

int main()
{
   double a, b, c,d;
   cin >> a >> b >> c >> d;
   for(double i = -100;i <= 100; i+=0.001)
   {
      double j = i + 0.001 , y =a * i *i *i+b * i * i + c * i + d,x = a * j * j * j + b * j * j + c * j + d;
      if(y >= 0 && x <= 0 || y<=0 && x >= 0 )
	  {
         double ans=(i + j) / 2;
         printf("%.2lf ",ans);
      }
   }
   return 0;
}

有一行敲得很累，所以请点个赞谢谢！！

#include<bits/stdc++.h> 
using namespace std;
double minn=0.0000001;
double m,a,b,c,d,e;
int k=1;
double f1(double x){
	return a*x*x*x+b*x*x+c*x+d;
}
double f2(){
	double l=-100;
	double r=100;
	for(double i=-100;i<100;i+=1.00000001){
		l=i;
		r=i+1;
		if(f1(l)*f1(r)>0){
			continue;
		}
		while(r-l>minn){
		m=(l+r)/2;
		if(f1(m)*f1(l)>0){
			l=m;
		}
		else{
			r=m;
		}
	}
	cout<<setprecision(2)<<fixed<<r<<" ";
	}
}
int main(){
	cin>>a>>b>>c>>d;
	f2();
}