poj Fishnet

http://poj.org/problem?id=1408

 #include<cstdio>
 #include<cstring>
 #include<cmath>
 #include<algorithm>
 #define maxn 1000
 using namespace std;

 const double eps=1e-;
 const double pi=acos(-1.0);

 int cmp(double x)
 {
     if(fabs(x)<eps) return ;
     if(x>) return ;
     return -;
 }

 struct point
 {
     double x,y;
     point(){}
     point(double a,double b):x(a),y(b) {}
     friend point operator -(const point &a,const point &b){
         return point(a.x-b.x,a.y-b.y);
     }
     friend point operator * (const point &a,const double b)
     {
         return point (a.x*b,a.y*b);
     }
     friend point operator / (const point &a,const double b)
     {
         return point(a.x/b,a.y/b);
     }
 };
 struct line
 {
     point a,b;
     line(){}
     line(point x,point y):a(x),b(y){}
 };
 inline double sqr(double x)
 {
     return x*x;
 }
 double det(point a,point b)
 {
     return a.x*b.y-a.y*b.x;
 }
 double dot(point a,point b)
 {
     return a.x*b.x+a.y*b.y;
 }
 double dist(point a,point b)
 {
     return sqrt(sqr(a.x-b.x)+sqr(a.y-b.y));
 }
 double area(point a[])
 {
     double sum=;
     a[]=a[];
     for(int i=; i<; i++)
     {
         sum+=det(a[i+],a[i]);
     }
     return sum/;
 }

 bool parallel (line a,line b)
 {
     return !cmp(det(a.a-a.b,b.a-b.b));
 }

 bool line_make_point(line a,line b,point &res)
 {
     if(parallel(a,b)) return false;
     double s1=det(a.a-b.a,b.b-b.a);
     double s2=det(a.b-b.a,b.b-b.a);
     res=(a.b*s1-a.a*s2)/(s1-s2);
     return true;
 }

 int main()
 {
    int n;
    double a[maxn],b[maxn],c[maxn],d[maxn];
    point m[][];
    line l[][];
    while(scanf("%d",&n)&&n){
        double max1=eps;
         for(int i=; i<=n; i++){
            scanf("%lf",&a[i]);
            m[][i].x=a[i];
            m[][i].y=;

         }
         m[][].x=;
         m[][].y=;
         m[][n+].x=;
         m[][n+].y=;
         for(int j=; j<=n; j++){
            scanf("%lf",&b[j]);
            m[n+][j].x=b[j];
            m[n+][j].y=;
         }
         m[n+][].x=;
         m[n+][].y=;
         m[n+][n+].x=;
         m[n+][n+].y=;
         for(int k=; k<=n; k++){
            scanf("%lf",&c[k]);
            m[k][].y=c[k];
            m[k][].x=;
         }
         for(int t= ; t<=n; t++){
            scanf("%lf",&d[t]);
            m[t][n+].x=;
            m[t][n+].y=d[t];
         }
         for(int i=; i<=n; i++)
         {
             l[i][].a=m[][i];
             l[i][].b=m[n+][i];
         }
         for(int i=; i<=n; i++)
         {
             l[][i].a=m[i][];
             l[][i].b=m[i][n+];
         }
         for(int i=; i<=n; i++)
         {
             for(int j=; j<=n; j++)
             {
                 point res;
                 line_make_point(l[][i],l[j][],res);
                 m[i][j]=res;
             }
         }
         for(int i=; i<=n+; i++)
         {
             for(int j=; j<=n+; j++)
             {
                 point s[];
                 s[]=m[i-][j-];
                 s[]=m[i-][j];
                 s[]=m[i][j];
                 s[]=m[i][j-];
                 max1=max(max1,area(s));
                 //printf("%.6lf ",area(s));
             }
             //printf("\n");
         }
         printf("%.6lf\n",max1);
    }
    return ;
 }