The 36th ACM/ICPC Asia Regional Chengdu Site 1003 Regular Polygon

比赛时悲剧的没做出来，赛后终于解决了，革命尚未成功，同志仍需努力呀！

#include <iostream>#include <math.h>#include <stdio.h>using namespace std;const int N=102;double pi=acos(-1);const double eps=1e-8;double dis[N];int n;double solve (double l,double r){    if(fabs(l-r)<eps)return -1;    double mid=(l+r)/2;    int i;    double ang=0;    for(i=0;i<n;i++)    {        if(mid-(dis[i]+dis[i+1])>eps)return solve(l,mid);//边太大，构不成三角形        if(mid-fabs(dis[i]-dis[i+1])<eps)return solve(mid,r);//边太小，构不成三角形        double cs=(dis[i]*dis[i]+dis[i+1]*dis[i+1]-mid*mid)/(2*dis[i]*dis[i+1]);        ang+=acos(cs);    }    if(fabs(ang-2*pi)<eps)return mid;//等于360度    else if(ang-2*pi>eps)return solve(l,mid);//大于360度，说明mid太大    else return solve(mid,r);//小于360度说明mid太小    return -1;}int main(){    int t;    cin>>t;    int ca=1;    while(t--)    {        cin>>n;        int i;        for(i=0;i<n;i++)        {            scanf("%lf",&dis[i]);        }        dis[n]=dis[0];        double ans=solve (0,20000);        if(ans==-1)printf("Case %d: impossible\n",ca);        else printf("Case %d: %.3lf\n",ca,ans);        ca++;    }    return 0;}/*233.0 4.0 5.031.0 2.0 3.0*/