UVA 10242 Fourth Point

题意：给你平行四边形两条边的顶点，让你求第四个点。

思路：要找到俩边的公共点，然后向量运算。

AC代码：

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<iostream>
#include<cstring>
using namespace std;
typedef long long ll;
const double eps = 1e-;
int sgn(double x)
{
    if(fabs(x) < eps) return ;
    else return x <  ? - : ;
}
struct Point{
    double x, y;
    Point(){}
    Point(double _x, double _y){
        x = _x, y = _y;
    }
    bool operator == (Point b) const{
        return sgn(x - b.x) ==  && sgn(y - b.y) == ;
    }
    bool operator < (Point b)const{
        return sgn(x - b.x) ==  ? sgn(y - b.y < ) : x < b.x;
    }
    Point operator - (const Point &b)const{
        return Point(x - b.x, y - b.y);
    }
    //叉积
    double operator ^(const Point &b){
        return x * b.y - y * b.x;
    }
    //点积
    double operator *(const Point &b){
        return x * b.x + y * b.y;
    }
    double len(){
        return hypot(x, y);
    }
    double len2(){
    return x * x + y * y;
    }
    double distant(Point p){
        return hypot(x - p.x, y - p.y);
    }
    Point operator + (const Point &b)const{
        return Point (x + b.x, y + b.y);
    }
    Point operator * (const double &k)const{
        return Point(x * k, y * k);
    }
    Point operator / (const double &k)const{
        return Point(x / k, y / k);
    }
};
int main()
{
    double x1, x2, x3, x4, y1, y2, y3, y4;
    Point p[], c;
    while(~scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4))
    {
        p[] = Point(x1,y1);
        p[] = Point(x2,y2);
        p[] = Point(x3,y3);
        p[] = Point(x4,y4);
        sort(p , p + );
        if(p[] == p[])
        {
            c = p[] + p[] - p[];
        }
        if(p[] == p[])
        {
            c = p[] + p[] - p[];
        }
        if(p[] == p[])
        {
            c = p[] + p[] - p[];
        }
        printf("%.3f %.3f\n",c.x, c.y);
    }
    return ;
}