常规而又直接的计算几何判断直线是否相交题目，如果相交，求相交的交点
在这里要注意两点：
1、scanf()输入double型数据要用%lf
2、用一般式（Ax+By+C=0）判断的直线时候记住重合的充要条件是 A2B1-A1B2==0 && A1C2-C1A2==0 && B1C2-C1B2==0（初高中的几何知识全还给老师了，(-_-)）
我犯错的时候是用A2B1-A1B2==0 && (A1C2-C1A2==0) ||( B1C2-C1B2==0)判断的，WA两次
#include<iostream> #include<cstdio> struct POINT { double x; double y; POINT():x(0),y(0) {}; POINT(double _x_,double _y_):x(_x_),y(_y_) {}; }; struct LINE { POINT a; POINT b; LINE() {}; LINE(POINT _a_,POINT _b_):a(_a_),b(_b_) {}; }; void Coeefficient(const LINE & L,double &A,double &B,double &C) { A = L.b.y-L.a.y; B = L.a.x-L.b.x; C = L.b.x*L.a.y-L.a.x*L.b.y; } POINT Intersection(const LINE &A,const LINE &B) { double A1,B1,C1; double A2,B2,C2; Coeefficient(A,A1,B1,C1); Coeefficient(B,A2,B2,C2); POINT I(0,0); I.x = - (B2*C1-B1*C2) / (A1*B2-A2*B1); I.y = (A2*C1-A1*C2) / (A1*B2-A2*B1); return I; } bool IsParallel(const LINE &A,const LINE &B) { double A1,B1,C1; double A2,B2,C2; Coeefficient(A,A1,B1,C1); Coeefficient(B,A2,B2,C2); //共线不算平行 /* return (A1*B2==A2*B1)&&((A1*C2!=A2*C1)||(B1*C2!=B2*C1));*/ //共线算平行 return (A1*B2==A2*B1); } bool IsPa(const LINE &A,const LINE &B) { double A1,B1,C1; double A2,B2,C2; Coeefficient(A,A1,B1,C1); Coeefficient(B,A2,B2,C2); return (A1*B2==A2*B1)&&(A1*C2==A2*C1)&&(B1*C2==B2*C1); } bool IsIntersect(const LINE &A,const LINE &B) { return !IsParallel(A,B); } int main() { //freopen("in.txt","r",stdin); int n; scanf("%d",&n); printf("INTERSECTING LINES OUTPUT\n"); while(n--) { LINE A,B; scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&A.a.x,&A.a.y,&A.b.x,&A.b.y,&B.a.x,&B.a.y,&B.b.x,&B.b.y); if(IsIntersect(A,B)) { POINT I=Intersection(A,B); printf("POINT %.2f %.2f\n",I.x,I.y); } else if(IsPa(A,B)) { printf("LINE\n"); } else printf("NONE\n"); } printf("END OF OUTPUT\n"); return 0; }
