题目：http://poj.org/problem?id=1408
题意：
一个1X1的正方形，每条边上有n个不同的点（不包括顶点），并给出它们的坐标。现在把对边相对应的点相连，将正方形分割成(n+1)*(n+1)个小四边形。问最大的四边形的面积是多少。
直接贴代码好了。。。
View Code
1 #include <iostream> 2 #include<cstdio> 3 #include<cmath> 4 using namespace std; 5 typedef struct node 6 { 7 double x,y; 8 }point; 9 point map[36][36]; 10 int n; 11 double det(double x1,double y1,double x2,double y2) 12 { 13 return x1*y2-x2*y1; 14 } 15 double cross(point a,point b,point c,point d) 16 { 17 return det(b.x-a.x,b.y-a.y,d.x-c.x,d.y-c.y); 18 } 19 point jiaodian(point a,point b,point c,point d) 20 { 21 point h; 22 double area1=cross(a,b,a,c); 23 double area2=cross(a,b,a,d); 24 25 h.x=(area2*c.x-area1*d.x)/(area2-area1); 26 h.y=(area2*c.y-area1*d.y)/(area2-area1); 27 28 return h; 29 } 30 double area(point a,point b,point c,point d) 31 { 32 double a2,a1; 33 a1=fabs(0.5*cross(a,b,a,c)); 34 a2=fabs(0.5*cross(a,b,a,d)); 35 return a1+a2; 36 } 37 int main() 38 { 39 while(scanf("%d",&n)!=EOF) 40 { 41 if(n==0) 42 break; 43 int i,j; 44 for(i=1;i<=n;i++) 45 { 46 scanf("%lf",&map[0][i].x); 47 map[0][i].y=0; 48 }//a 49 for(i=1;i<=n;i++) 50 { 51 scanf("%lf",&map[n+1][i].x); 52 map[n+1][i].y=1; 53 }//b 54 for(i=1;i<=n;i++) 55 { 56 scanf("%lf",&map[i][0].y); 57 map[i][0].x=0; 58 }//c 59 for(i=1;i<=n;i++) 60 { 61 scanf("%lf",&map[i][n+1].y); 62 map[i][n+1].x=1; 63 }//d 64 map[0][0].x=map[0][0].y=0; 65 map[n+1][n+1].x=map[n+1][n+1].y=1; 66 map[n+1][0].y=1; 67 map[n+1][0].x=0; 68 map[0][n+1].y=0; 69 map[0][n+1].x=1; 70 for(i=1;i<=n;i++) 71 { 72 point p; 73 for(j=1;j<=n;j++) 74 { 75 p=jiaodian(map[0][j],map[n+1][j],map[i][0],map[i][n+1]); 76 map[i][j]=p; 77 } 78 79 } 80 /*for(i=0;i<=n+1;i++) 81 { 82 for(j=0;j<=n+1;j++) 83 { 84 printf("(%f,%f) ",map[i][j].x,map[i][j].y); 85 } 86 printf("\n"); 87 }*/ 88 double res=0; 89 for(i=0;i<=n;i++) 90 { 91 for(j=0;j<=n;j++) 92 { 93 double uu; 94 uu=area(map[i][j],map[i+1][j+1],map[i+1][j],map[i][j+1]); 95 if(uu>res) 96 res=uu; 97 } 98 } 99 printf("%.6f\n",res); 100 } 101 return 0; 102 }