Poj 1269 Intersecting Lines （直线交点）

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题意：分别给出两条直线上的两个点，求这两条直线的位置关系，如果相交则输出交点。

思路：利用向量。

#include <cmath>#include <cstdio>#include <cstring>struct Point  //点,向量{double x,y;Point(){}Point(double _x,double _y){x=_x;y=_y;}Point operator-(const Point &b)const{return Point(x-b.x,y-b.y);}    Point operator+(const Point &b){return Point(x+b.x,y+b.y);}    Point operator*(const double k)  //数乘    {return Point(x*k,y*k);}double operator*(const Point a)  //点乘    {return x*a.x+y*a.y;}    double operator^(const Point a)  //叉乘    {return x*a.y-y*a.x;}};int DB (double x){if(fabs(x)<1e-6) return 0;return x>0?1:-1;}//求两条直线交点,输入直线1上两点A,B，直线2上两点C,D，点ans保存交点//返回0表示平行//返回1表示重合//返回2表示相交,此时ans会保存交点int Line_cross_line (Point A,Point B,Point C,Point D,Point& ans){    Point AB=B-A;Point AC=C-A;Point AD=D-A;Point CD=D-C;if (DB(AB^CD)==0)   //叉乘    {        if (DB(AC^AD)==0)return 0;        else return 1;    }    double d=sqrt(AB.x*AB.x+AB.y*AB.y);    double s1=AC^AB,s2=AB^AD;    double k=s1/(s1+s2);    ans=C+CD*k;    return 2;}int main (){#ifdef ONLINE_JUDGE#elsefreopen("read.txt","r",stdin);#endifint T;scanf("%d",&T);printf("INTERSECTING LINES OUTPUT\n");while (T--){    Point A,B,C,D,ans;    scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&A.x,&A.y,&B.x,&B.y,&C.x,&C.y,&D.x,&D.y);    int flag=Line_cross_line(A,B,C,D,ans);    if (flag==0) printf("LINE\n");    else if (flag==1) printf("NONE\n");    else printf("POINT %.2lf %.2lf\n",ans.x,ans.y);}printf("END OF OUTPUT\n");return 0;}