【二分】hdu 4033

#define LL long long#define PI acos(-1.0)#define N  105#define MAX INT_MAX#define MIN INT_MIN#define eps 1e-8double a[N];int n;double maxm;double cal(double aa,double bb,double cc ){    return (aa*aa+bb*bb-cc*cc)/(2*aa*bb);}int gao(double x){    int i,j;    double res=0.0;    for(i=0;i<n;i++){        if(a[i]+a[(i+1)%n]<=x)return 0;//big        if(fabs( a[i]-a[(i+1)%n] )>=x)return -1;//small        double tmp=cal(a[i],a[(i+1)%n],x);        res+=acos(tmp);    }    if(fabs(res-2*PI)<eps)return 1;//equal    else    if(res<2*PI)return -1;//small    else    if(res>2*PI)    return 0;//big}int main(){    int t;    scanf("%d",&t);    int ca=1;    while(t--){        scanf("%d",&n);        int i,j;        maxm=0.0;        for(i=0;i<n;i++){            scanf("%lf",&a[i]);            if(a[i]>maxm)maxm=a[i];        }        double l=0,r=2*maxm,mid;        bool flag=0;        while((r-l)>eps){            mid=(l+r)/2.0;//cout<<mid<<endl;            int tag=gao(mid);            if(tag==1){                flag=1;                break;            }            if(tag==0){//big                r=mid;            }            else{//small                l=mid;            }        }        printf("Case %d: ",ca++);        if(!flag)printf("impossible\n");        else{            printf("%.3f\n",mid);        }    }    return 0;}