bzoj1536: [POI2005]Akc- Special Forces Manoeuvres

#include<cstdio>
#include<cmath>
typedef long double ld;
const ld _0=1e-7;
ld x[2007],y[2007],r[2007];
void get(int id,ld X,ld&a,ld&b){
    X-=x[id];
    X=std::sqrt(r[id]*r[id]-X*X);
    a=y[id]-X-_0;
    b=y[id]+X+_0;
}
ld dis(int a,int b){
    ld X=x[a]-x[b],Y=y[a]-y[b];
    return sqrt(X*X+Y*Y);
}
ld cal(int a,int b){
    return x[a]+(x[b]-x[a])*r[a]/dis(a,b);
}
bool chk(int n){
    ld L=x[1]-r[1],R=x[1]+r[1],M;
    for(int i=2;i<=n;++i){
        if(x[i]-r[i]<L)L=x[i]-r[i];
        if(x[i]+r[i]>R)R=x[i]+r[i];
    }
    while(1){
        M=(L+R)/2;
        bool dl=0,dr=0;
        for(int i=1;i<=n;++i){
            if(x[i]+r[i]<M)dl=1;
            if(x[i]-r[i]>M)dr=1;
        }
        if(dl){
            if(dr)return 0;
            R=M;
        }else if(dr)L=M;
        else{
            ld y1,y2,a1,a2;
            get(1,M,y1,y2);
            for(int i=2;i<=n;++i){
                get(i,M,a1,a2);
                if(a1>y1)y1=a1;
                if(a2<y2)y2=a2;
                if(y1>y2){
                    for(int j=1;j<i;++j){
                        get(j,M,y1,y2);
                        if(y1>a2||y2<a1){
                            if(dis(i,j)>r[i]+r[j]+_0)return 0;
                            ld xm=cal(i,j);
                            if(xm>R||xm<L)return 0;
                            if(M<xm)L=M;
                            else R=M;
                            goto out;
                        }
                    }
                }
            }
            return 1;
        }
        out:;
    }
}
int main(){
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;++i)scanf("%llf%llf%llf",x+i,y+i,r+i),r[i]+=_0;
    if(chk(n))return puts("NIE"),0;
    int L=1,R=n,M;
    while(L<R){
        M=L+R>>1;
        if(chk(M))L=M+1;
        else R=M;
    }
    return printf("%d",L),0;
}