HDU3934+凸包

題意:給定一些點，求最大三角形面積

/*凸包*/#include<stdio.h>#include<string.h>#include<stdlib.h>#include<algorithm>#include<iostream>#include<queue>//#include<map>#include<math.h>using namespace std;typedef long long ll;//typedef __int64 int64;const int maxn = 1000005;const int inf = 0x7fffffff;const double pi=acos(-1.0);struct node{    int x,y;    bool operator <( const node & p ) const {        return y<p.y||(y==p.y&&x<p.x);    }};node pnt[ maxn ],res[ maxn ];int cross ( node sp,node ep,node op ){    return ( sp.x-op.x )*( ep.y-op.y )-( sp.y-op.y )*( ep.x-op.x );}int dis2( node a,node b ){    return ( a.x-b.x )*( a.x-b.x )+( a.y-b.y )*( a.y-b.y );}int garham( int n ){    int top=1;    sort( pnt,pnt+n );    if( n==0 ) return 0;    else res[ 0 ]=pnt[ 0 ];    if( n==1 ) return 1;    else res[ 1 ]=pnt[ 1 ];    if( n==2 ) return 2;    else res[ 2 ]=pnt[ 2 ];        for( int i=2;i<n;i++ ){        while( top>0&&cross( res[ top-1 ],pnt[ i ],res[ top ] )>=0 )            top--;        res[ ++top ]=pnt[ i ];    }        int len=top;    res[ ++top ]=pnt[ n-2 ];    for( int i=n-3;i>=0;i-- ){        while( top!=len&&cross( res[ top-1 ],pnt[ i ],res[ top ] )>=0 )            top--;        res[ ++top ]=pnt[ i ];    }        return top;}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=garham( 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++ ){                    ans=max( ans,( 0.5*cross( res[ j ],res[ k ],res[ i ] ) ) );                }            }        }        printf("%.2lf\n",ans);    }    return 0;}