三分 HDOJ 3714 Error Curves

题目传送门

/*
    三分：凹(凸)函数求极值
*/
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
using namespace std;

const int MAXN = 1e4 + 10;
const int INF = 0x3f3f3f3f;
const double EPS = 0.0000000001;
struct F    {
    double a, b, c;
}f[MAXN];
int n;

double cal(double x)    {
    double res = -INF;
    for (int i=1; i<=n; ++i)    {
        res = max (res, f[i].a * x * x + f[i].b * x + f[i].c);
    }
    return res;
}

int main(void)  {       //HDOJ 3714 Error Curves
    //freopen ("HDOJ_3714.in", "r", stdin);

    int T;  scanf ("%d", &T);
    while (T--) {
        scanf ("%d", &n);
        for (int i=1; i<=n; ++i)    {
            scanf ("%lf%lf%lf", &f[i].a, &f[i].b, &f[i].c);
        }

        double l = 0.0, r = 1000.0;
        for (int i=1; i<=100; ++i)  {
            double mid = (l + r) / 2;
            double rmid = (mid + r) / 2;
            if (cal (mid) < cal (rmid)) r = rmid;
            else    l = mid;
        }
        printf ("%.4f
", cal (r));
    }

    return 0;
}