POJ 1654

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题目：

求n个点的多边形的有向面积，算法见书P258

#include <iostream>#include<cstdio>#include<cstring>#define M 1000000using namespace std;char s[M+5];int stepx[10]={  0,-1,0,1,-1,0,1,-1,0,1};int stepy[10]={  0,-1,-1,-1,0,0,0,1,1,1};struct Point{    double x,y;    Point(double x=0,double y=0):x(x),y(y) {}}p[3];typedef Point Vector;Vector operator - (Point A,Point B) {return Vector(A.x-B.x,A.y-B.y);}double Cross(Vector A,Vector B) {return A.x*B.y-A.y*B.x;}double Area(Point *p,int n){    double area=0;    for(int i=1;i<=n-1;i++)        {            area+=Cross(p[i]-p[0],p[i+1]-p[0]);   //         printf("area=%.2f\n",area);        }    if(area<0) area=-area;    return area/2;}int main(){    int ncase;    int len,n;    double area;    scanf("%d",&ncase);    while(ncase--)    {        scanf("%s",s);        len=strlen(s);        n=len-2;        if(n<=1) puts("0");        else{            area=0;            p[1].x=0+stepx[s[0]-'0'],            p[1].y=0+stepy[s[0]-'0'];            for(int i=2;i<=n;i++)               {                p[2].x=p[1].x+stepx[s[i-1]-'0'],                p[2].y=p[1].y+stepy[s[i-1]-'0'];                area+=Cross(p[1]-p[0],p[2]-p[0]);              //  printf("area=%.2f\n",area);                p[1]=Point(p[2].x,p[2].y);               }            if(area<0) area=-area;             long long a=(long long)(area); // 要用longlong或者 __int64          //  printf("a=%d\n",a);            if( a%2) printf("%lld.5\n",a/2);            else printf("%lld\n",a/2);        }    }    return 0;}

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