UVa 378 - Intersecting Lines

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题目：给你平面上的两条直线，判断两直线关系，如果相交求交点。

分析：计算几何。利用斜率判断平行，然后利用叉乘判断共线，最后求交点。

说明：注意精度，又是好久没刷题╮(╯▽╰)╭。

#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <cmath>

using namespace std;

//定义点结构 
typedef struct pnode  
{  
    double x,y,d;  
    pnode( double a, double b ) {x = a;y = b;}  
    pnode(){};  
}point;  

//定义线结构 
typedef struct lnode  
{  
    double x,y,dx,dy;  
    lnode(){}  
    lnode(double X, double Y, double DX, double DY) {  
        x = X;y = Y;dx = DX;dy = DY;  
    }  
}line; 

//叉乘 
double crossproduct(point a, point b, point c)  
{  
    return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);  
}

//不平行线段交点 
point cross(line l1, line l2)
{
	double A1 = l1.dy,B1 = l1.dx,C1 = l1.dy*l1.x-l1.dx*l1.y;
	double A2 = l2.dy,B2 = l2.dx,C2 = l2.dy*l2.x-l2.dx*l2.y;
	double x = (C1*B2-C2*B1)/(A1*B2-A2*B1);
	double y = (C1*A2-C2*A1)/(A1*B2-A2*B1);
	return point(x, y); 
} 

int main()
{
	point p1,p2,p3,p4;
	int n;
	while (~scanf("%d",&n)) {
		printf("INTERSECTING LINES OUTPUT\n");
		while (n --) {
			scanf("%lf%lf%lf%lf%lf%lf%lf%lf",				&p1.x,&p1.y,&p2.x,&p2.y,&p3.x,&p3.y,&p4.x,&p4.y);
			line l1 = line(p1.x, p1.y, p1.x-p2.x, p1.y-p2.y);
			line l2 = line(p3.x, p3.y, p3.x-p4.x, p3.y-p4.y);
			
			if ((p4.x-p3.x)*(p2.y-p1.y) == (p2.x-p1.x)*(p4.y-p3.y)) {
				if (crossproduct(p1, p2, p3)  == 0)
					printf("LINE\n");
				else printf("NONE\n");
			}else {
				point cp = cross(l1, l2);
				printf("POINT %.2lf %.2lf\n",cp.x,cp.y);
			}
		}
		printf("END OF OUTPUT\n");
	}
	
    return 0;
}

UVa 378 - Intersecting Lines

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原文地址：http://blog.csdn.net/mobius_strip/article/details/45148557