叉积_判断点是否在线段上

//判断点是否在直线上
//如果在直线上，那么这个点和线段两端点构成的向量的叉积一定==0
//也就是两个向量共线(obviously)
//那么如果坐标还在两端点之间的话就是在线段上了 

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
using namespace std;

const int N=1e5+5;
const double eps=1e-8;

inline int read()
{
    char c=getchar();int num=0;
    for(;!isdigit(c);c=getchar());
    for(;isdigit(c);c=getchar())
        num=num*10+c-'0';
    return num;
}

struct Point
{
    double x,y;
    Point(double x=0,double y=0)
    {
        this->x=x,this->y=y;
    }
    Point operator - (const Point &a)
    {
        return Point(this->x-a.x,this->y-a.y);
    }
    double operator * (const Point &a)
    {
        return this->x*a.y-this->y*a.x;
    }
}Q,P1,P2;

int main()
{
    scanf("%lf%lf%lf%lf%lf%lf",&Q.x,&Q.y,&P1.x,&P1.y,&P2.x,&P2.y);
    if(fabs((Q-P1)*(Q-P2))<eps&&min(P1.x,P2.x)-eps<=Q.x&&max(P1.x,P2.x)+eps>=Q.x&&min(P1.y,P2.y)-eps<=Q.y&&max(P1.y,P2.y)+eps>=Q.y)
        puts("Yes");
    else
        puts("No");
    return 0;
}