poj 1569

# include <stdio.h>
# include <stdlib.h>
struct node
{
    int x,y;
}vex[20];
int cross(node a,node b,node c)
{
    return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);
}
int area(node a,node b,node c)
{
    return abs(cross(a,b,c));
}
int solve(int i,int j,int k,int m)
{
    if(cross(vex[i],vex[j],vex[m])<=0&&cross(vex[j],vex[k],vex[m])<=0&&cross(vex[k],vex[i],vex[m])<=0||
       cross(vex[i],vex[j],vex[m])>=0&&cross(vex[j],vex[k],vex[m])>=0&&cross(vex[k],vex[i],vex[m])>=0)
    return 1;
    else
        return 0;
}
int main ()
{
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        if(n==0)  break;
        char c;
        for(int i=0;i<n;i++)
        {
            c=getchar();
            c=getchar();
            scanf("%d%d",&vex[i].x,&vex[i].y);
        }
        int mark=0,mmax=0,ans,ii,jj,kk;
        for(int i=0;i<n;i++)
            for(int j=i+1;j<n;j++)
                for(int k=j+1;k<n;k++)
                {
                    mark=0;
                    if(cross(vex[i],vex[j],vex[k])==0)
                        continue;
                    for(int m=0;m<n;m++)
                    {
                        if(m==i||m==j||m==k) continue;
                        if(solve(i,j,k,m))
                            {mark=1;break;}
                    }
                    //printf("mark===%d\n",mark);
                    if(mark==1)  continue;
                    ans=area(vex[i],vex[j],vex[k]);
                    //printf("ans=====%d\n",ans);
                    if(ans>mmax)
                    {
                        mmax=ans;
                        ii=i;jj=j;kk=k;
                    }
                }
        printf("%c%c%c\n",ii+'A',jj+'A',kk+'A');
    }
    return 0;
}

注意判斷點在三角形內部的判斷