二分法求三次方程的根

#include "stdio.h"
#define f(x) a*x*x*x+b*x*x+c*x+d

int main()
{
freopen("in.txt", "r", stdin);
int a, b, c, d;
double x1, x2, x, y1, y2, y;
while (scanf("%d%d%d%d", &a, &b, &c, &d) != EOF)
{
x1 = -100;
x2 = 100;
x = (x1+x2) / 2;
while ((x2-x1) > 0.01)
{
y1 = f(x1);
y2 = f(x2);
y = f(x);
if (y1 == 0)
{
x = x1;
break;
}
if (y2 == 0)
{
x = x2;
break;
}
if (y1*y < 0)
{
x2 = x;
x = (x1+x2) / 2;
}
else
{
x1 = x;
x = (x1+x2) / 2;
}
}
printf("%.f\n", x);
}
return 0;
}