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 + ;
 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=; 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=; 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=; i<=; ++i)  {
             double mid = (l + r) / ;
             double rmid = (mid + r) / ;
             if (cal (mid) < cal (rmid)) r = rmid;
             else    l = mid;
         }
         printf ("%.4f\n", cal (r));
     }

     return ;
 }