HDU2202 凸包

凸包算法+枚举

 /*
 凸包
 顺时针！！！！
 */
 #include<stdio.h>
 #include<string.h>
 #include<stdlib.h>
 #include<algorithm>
 #include<iostream>
 #include<queue>
 #include<stack>
 #include<math.h>
 #include<map>
 using namespace std;
 const int maxn = 50005;
 struct node{
     int x,y;
 };
 bool operator < ( const node &l,const node &r ){
         return l.y<r.y||( l.y==r.y&&l.x<r.x );
 }
 node pnt[ maxn ],res[ maxn ];
int mult( node sp,node ep,node op ){
     return (sp.x - op.x) * (ep.y - op.y)-(ep.x - op.x) * (sp.y - op.y);
 }
 /*
 ep
 |
 |
 op----sp
 ( from sp to ep )
 */
 int graham( int n ){
     int i, len, k = 0;
     int top = 1;
     sort(pnt, pnt + n);
     if (n == 0) return 0; res[0] = pnt[0];
     if (n == 1) return 1; res[1] = pnt[1];
     if (n == 2) return 2; res[2] = pnt[2];
     for (i = 2; i < n; i++) {
         while (top && mult(res[ top ], pnt[ i ], res[top-1])>=0 )//( cross ： from top to i )
             top--;
         res[++top] = pnt[i];
     }
     len = top; res[++top] = pnt[n - 2];
     for (i = n - 3; i >= 0; i--) {
         while (top!=len && mult(res[ top ], pnt[ i ], res[top-1])>=0 ) 
         top--;
         res[++top] = pnt[i];
     }
     return top; // 返回凸包中点的个数
 }
 int main(){
     int n;
     while( scanf("%d",&n)!=EOF ){
         for( int i=0;i<n;i++ )
             scanf("%d%d",&pnt[i].x,&pnt[i].y);
         int cnt=graham( n );
         double ans=0;
         for( int i=0;i<cnt;i++ ){
             for( int j=i+1;j<cnt;j++ ){
                 for( int k=j+1;k<cnt;k++ ){
                     double tmp=mult( res[i],res[k],res[j] );
                     ans=max( ans,tmp );
                 }
             }
         }
         ans*=0.5;
         printf("%.2lf\n",ans);
     }
     return 0;
 }