BestCoder Round #18(hdu5105)Math Problem(高中数学)

最大值无非就是在两个端点或极值点处取得。

我注意讨论了a=0和b=0，却忽略了极值点可能不在L到R的范围内这一问题。被Hack了。

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<cmath>
#include<map>
#include<set>
#include<vector>
#include<algorithm>
#include<stack>
#include<queue>
#include<cctype>
#include<sstream>
using namespace std;
#define pii pair<int,int>
#define LL long long int
const int eps=1e-8;
const int INF=1000000000;
const int maxn=10000+10;
double a,b,c,d,L,R,ans;
double f(double x)
{
    return fabs(a*x*x*x+b*x*x+c*x+d);
}
int main()
{
    //freopen("in10.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    while(scanf("%lf%lf%lf%lf%lf%lf",&a,&b,&c,&d,&L,&R)==6)
    {
        if(a!=0)
        {
            if(a<0)
            {
                a=-a;
                b=-b;
                c=-c;
                d=-d;
            }
            double pan=b*b-3*a*c;
            if(pan<=0)
            {
                printf("%.2f
",max(f(L),f(R)));
            }
            else
            {
                double x1=(-sqrt(pan)-b)/(3*a);
                double x2=(sqrt(pan)-b)/(3*a);
                if(x1>=L&&x1<=R&&x2>=L&&x2<=R)
                printf("%.2f
",max(max(f(L),f(R)),max(f(x1),f(x2))));
                else if(x1>=L&&x1<=R)
                    printf("%.2f
",max(f(L),max(f(R),f(x1))));
                else if(x2>=L&&x2<=R)
                    printf("%.2f
",max(f(L),max(f(R),f(x2))));
                else printf("%.2f
",max(f(L),f(R)));
            }
        }
        else
        {
            if(b==0) printf("%.2f
",max(f(L),f(R)));
            else
            {
                double z=(-c)/(2*b);
                if(z<=R&&z>=L)
                printf("%.2f
",max(f(L),max(f(R),f(z))));
                else printf("%.2f
",max(f(L),f(R)));
            }
        }
    }
    //fclose(stdin);
    //fclose(stdout);
    return 0;
}