hdu 5017 Ellipsoid(三分)

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>

using namespace std;
const double INF = 10000;
const double eps = 1e-9;
double a, b, c, d, e, f;

double get(double A, double B, double C) {
if (B * B - 4 * A * C < eps)
return INF;
return (sqrt(B * B - 4 * A * C) - B) / (2 * A);
}

double func (double x, double y) {
double z = get(c, e * x + d * y, a * x * x + b * y * y + f * x * y - 1);
return x * x + y * y + z * z;
}

double search (double x) {
double l = -INF, r = INF;
for (int i = 0; i < 200; i++) {
double ll = l + (r - l) / 3;
double rr = r - (r - l) / 3;
if (func(x, ll) > func(x, rr))
l = ll;
else
r = rr;
}
return func(x, l);
}

double solve (double l, double r) {
for (int i = 0; i < 200; i++) {
double ll = l + (r - l) / 3;
double rr = r - (r - l) / 3;
if (search(ll) > search(rr))
l = ll;
else
r = rr;
}
return sqrt(search(l));
}

int main () {
while (scanf("%lf%lf%lf%lf%lf%lf", &a, &b, &c, &d, &e, &f) == 6) {
printf("%.5lf\n", solve(-INF, INF));
}
return 0;
}