poj1408---计算几何

//对n*n个四边形，求面积最大的那个的面积
#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#define eps 1e-8
#define dist(a,b) sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y))
#define cross(a,b,c) (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x)
#define dot(a,b,c) (b.x-a.x)*(c.x-a.x)+(b.y-a.y)*(c.y-a.y)
#define delt(a) fabs(a)<eps?0:a>0?1:-1
#define max(a,b) (a>b?a:b)
#define min(a,b) (a<b?a:b)
#define N 100
#define inf 1234567890
using namespace std;
struct TPoint
{
    double x,y;
    TPoint(){}
    TPoint(double _x,double _y):x(_x),y(_y){}
}pt[4][N];
int n,m,t;
void scan()
{
	int i;
	for(i=1;i<=n;i++) 
	{
		scanf("%lf",&pt[0][i].x);
		pt[0][i].y=0;
	}
	for(i=1;i<=n;i++)
	{
		scanf("%lf",&pt[1][i].x);
		pt[1][i].y=1;
	}
	for(i=1;i<=n;i++)
	{
		scanf("%lf",&pt[2][i].y);
		pt[2][i].x=0;
	}
	for(i=1;i<=n;i++)
	{
		scanf("%lf",&pt[3][i].y);
		pt[3][i].x=1;
	}
	n+=2;
	pt[0][0].x=0,pt[0][n-1].x=1,pt[0][0].y=pt[0][n-1].y=0;
	pt[1][0].x=0,pt[1][n-1].x=1,pt[1][0].y=pt[1][n-1].y=1;
	pt[2][0].y=0,pt[2][n-1].y=1,pt[2][0].x=pt[2][n-1].x=0;
	pt[3][0].y=0,pt[3][n-1].y=1,pt[3][0].x=pt[3][n-1].x=1;
}
TPoint segcross(TPoint a,TPoint b,TPoint c,TPoint d)
{
	TPoint tmp=TPoint(0,0);
	double s1=cross(a,c,d),s2=cross(b,c,d),s3=cross(c,a,b),s4=cross(d,a,b);
	int d1=delt(s1),d2=delt(s2),d3=delt(s3),d4=delt(s4);
	if(d1*d2<0&&d3*d4<0) 
	{
		tmp.x=(s2*a.x-s1*b.x)/(s2-s1);
		tmp.y=(s2*a.y-s1*b.y)/(s2-s1);
	}
	else if(d1==0) tmp=a;
	else if(d2==0) tmp=b;
	else if(d3==0) tmp=c;
	else tmp=d;
	return tmp;
}
double getarea(TPoint a,TPoint b,TPoint c,TPoint d)
{
	double s1=cross(a,b,c),s2=cross(a,c,d);
	return (fabs(s1)+fabs(s2))*0.5;
}
double solve(TPoint sts,TPoint ste,TPoint ens,TPoint ene)
{
	double maxarea=-1,area;
	int i,j,k;
	TPoint a,b,c,d;
	a=sts,b=ens;
	for(i=1;i<n;i++)
	{
		c=segcross(ens,ene,pt[2][i],pt[3][i]);
		d=segcross(sts,ste,pt[2][i],pt[3][i]);
		area=getarea(a,b,c,d);
		maxarea=max(area,maxarea);
		a=d,b=c;
	}
	return maxarea;
}
void solve()
{
	TPoint sts,ste,ens,ene;
	double area,maxarea=-1;
	for(int i=1;i<n;i++)
	{
		sts=pt[0][i-1],ste=pt[1][i-1];
		ens=pt[0][i],ene=pt[1][i];
		area=solve(sts,ste,ens,ene);
		maxarea=max(area,maxarea);
	}
	printf("%.6f\n",maxarea);
}
int main()
{
    while(scanf("%d",&n),n)
    {
		scan();
		solve();
    }
    return 0;
}