hdu 1756 Cubid's Arrow（判断点在多边形内）

题目链接：点击打开链接

判断点在多边形内模板题，刘汝佳白书模板。

方法是从此点向右引一条射线，判断多边形的边顺时针逆时针穿过此射线次数是否相等，相等则在外面。

注意代码中

    if(k>0&&d2>=0&&d1<0) wn++;
    if(k<0&&d2<0&&d1>=0) wn--; 此处d1,d2都只在一个等式中取等号，wa好几次

代码：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <vector>
#define eps 1e-8
using namespace std;

double fabs(double x){
    return x>0?x:-x;
}

struct Point{
    double x,y;
    Point(){}
    Point(double x,double y):x(x),y(y){}
    Point operator - (Point obj){
        return Point(x-obj.x,y-obj.y);
    }

};

vector <Point > polygon;
double Cross(Point a,Point b){
    return a.x*b.y-a.y*b.x;
}

int dcmp(double a){
    if(fabs(a)<eps) return 0;
    else if(a>0)return 1;
    else return -1;
}

double Dot(Point a,Point b){
    return a.x*b.x+a.y*b.y;
}

bool OnSegment(Point t,Point a,Point b){
    return Cross(a-t,b-t)==0&&Dot(a-t,b-t)<=0;
}

bool InPolygon(vector <Point> v,Point t){
    int wn=0;
    int n=v.size();
    for(int i=0;i<n;i++){
        if(OnSegment(t,v[i],v[(i+1)%n]))return 0;
        int k=dcmp(Cross(t-v[i],v[(i+1)%n]-v[i]));

        int d1=dcmp(v[(i+1)%n].y-t.y);
        int d2=dcmp(v[i].y-t.y);
        if(k>0&&d2>=0&&d1<0) wn++;
        if(k<0&&d2<0&&d1>=0) wn--;//d1,d2都只在一个等式中取等号
    }
    if(wn!=0)return 1;
    return 0;
}
int N,M;
int main(){
    while(~scanf("%d",&N)){
        polygon.clear();
        for(int i=0;i<N;i++){
            double x,y;
            scanf("%lf%lf",&x,&y);
            polygon.push_back(Point(x,y));
        }
        scanf("%d",&M);
        for(int j=0;j<M;j++){
            double x,y;
            scanf("%lf%lf",&x,&y);
            Point t(x,y);
            if(InPolygon(polygon,t)) printf("Yes\n");
            else printf("No\n");
        }
    }
    return 0;
}