poj-1408-计算两线相交点坐标模板

4个点
(a1,b1) (a2,b2)组成一条线
(c1,d1) (c2,d2)组成一条线
x= (
(a2-a1)(c2-c1)(d2-b2)
+(b2-b1)*(c2-c1)*a2
-(d2-d1)*(a2-a1)*c2
)
/
(
(b2-b1)*(c2-c1)
-(d2-d1)*(a2-a1)
);
y= (b2-b1)/(a2-a1)*(x-a2)+b2;

#include<cstdio>
#include<cmath>
using namespace std;
int n;
struct left{
  double x,y;
}l[100];
struct right{
   double x,y;
}r[100];
struct up{
   double x,y;
}u[100];
struct down{
   double x,y;
}d[100];
struct coor{
  double x,y;
}co[100];
void init()
{
     for(int i=1;i<=n;i++)
    {
      scanf("%lf",&d[i].x);
      d[i].y = 0.0;
    }
       d[0].x = 0.0;
       d[0].y = 0.0;
       d[n+1].x = 1.0;
       d[n+1].y = 0.0;
    for(int i=1;i<=n;i++)
    {
      scanf("%lf",&u[i].x);
      u[i].y = 1.0;
    }
       u[0].x = 0.0;
       u[0].y = 1.0;
       u[n+1].x = 1.0;
       u[n+1].y = 1.0;
      for(int i=1;i<=n;i++)
    {
      scanf("%lf",&l[i].y);
      l[i].x = 0.0;
    }
       l[0].y = 0.0;
       l[0].x = 0.0;
       l[n+1].y = 1.0;
       l[n+1].x = 0.0;
      for(int i=1;i<=n;i++)
    {
      scanf("%lf",&r[i].y);
      r[i].x = 1.0;
    }
       r[0].y = 0.0;
       r[0].x = 1.0;
       r[n+1].y = 1.0;
       r[n+1].x = 1.0;
}
void toCalCoor(int sym,int a,int b)
{
    co[sym].x = ((r[a].x-l[a].x)*(u[b].x-d[b].x)*(u[b].y-r[a].y)
                 +(r[a].y-l[a].y)*(u[b].x-d[b].x)*r[a].x
                 -(u[b].y-d[b].y)*(r[a].x-l[a].x)*u[b].x )
                 /
                 ((r[a].y-l[a].y)*(u[b].x-d[b].x)
                  -(u[b].y-d[b].y)*(r[a].x-l[a].x));
    co[sym].y = (r[a].y-l[a].y)/(r[a].x-l[a].x)*(co[sym].x-r[a].x)+r[a].y;
    //计算相交点的坐标
}
double toCalArea(int i,int j)
{
    double area = 0.0;
    toCalCoor(0,i,j);
    toCalCoor(1,i+1,j);
    toCalCoor(2,i+1,j+1);
    toCalCoor(3,i,j+1);
    for(int k=0;k<4;k++)
      area+=co[k].x*co[(k+1)%4].y-co[k].y*co[(k+1)%4].x;
    return fabs(area*0.5);
}
int main()
{
    while(scanf("%d",&n)&&n)
    {
        double ans = -1.0;
         init();
         for(int i=0;i<=n;i++)
           for(int j=0;j<=n;j++)
         {
             double s = toCalArea(i,j);
             if(ans<s)ans = s;
         }
         printf("%f\n",ans);
    }
    return 0;
}