简单几何(直线求交点) POJ 2074 Line of Sight

题目传送门

题意：从一条马路(线段)看对面的房子(线段)，问连续的能看到房子全部的最长区间

分析：自己的思路WA了：先对障碍物根据坐标排序，然后在相邻的障碍物的间隔找到区间，这样还要判断是否被其他障碍物遮挡住(哇

　　　　网上有很好的思路，先对每条线段找到阴影的端点，然后根据坐标排序，求和左端点的距离的最大值，这样省去线段相交的判断。

　　　　trick点应该就是障碍物的位置随意，可能在房子和马路的外面。

/************************************************
* Author        :Running_Time
* Created Time  :2015/11/2 星期一 20:33:38
* File Name     :POJ_2074.cpp
 ************************************************/

#include <cstdio>
#include <algorithm>
#include <iostream>
#include <sstream>
#include <cstring>
#include <cmath>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <bitset>
#include <cstdlib>
#include <ctime>
using namespace std;

#define lson l, mid, rt << 1
#define rson mid + 1, r, rt << 1 | 1
typedef long long ll;
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
const double EPS = 1e-10;
const double PI = acos (-1.0);
int dcmp(double x)  {       //三态函数，减少精度问题
    if (fabs (x) < EPS) return 0;
    else    return x < 0 ? -1 : 1;
}
struct Point    {       //点的定义
    double x, y;
    Point () {}
    Point (double x, double y) : x (x), y (y) {}
    Point operator + (const Point &r) const {       //向量加法
        return Point (x + r.x, y + r.y);
    }
    Point operator - (const Point &r) const {       //向量减法
        return Point (x - r.x, y - r.y);
    }
    Point operator * (double p) const {       //向量乘以标量
        return Point (x * p, y * p);
    }
    Point operator / (double p) const {       //向量除以标量
        return Point (x / p, y / p);
    }
    bool operator < (const Point &r) const {       //点的坐标排序
        return x < r.x || (x == r.x && y < r.y);
    }
    bool operator == (const Point &r) const {       //判断同一个点
        return dcmp (x - r.x) == 0 && dcmp (y - r.y) == 0;
    }
};
typedef Point Vector;       //向量的定义
Point read_point(void)   {      //点的读入
    double x, y;
    scanf ("%lf%lf", &x, &y);
    return Point (x, y);
}
double dot(Vector A, Vector B)  {       //向量点积
    return A.x * B.x + A.y * B.y;
}
double cross(Vector A, Vector B)    {       //向量叉积
    return A.x * B.y - A.y * B.x;
}
double polar_angle(Vector A)  {     //向量极角
    return atan2 (A.y, A.x);
}
double length(Vector A) {       //向量长度，点积
    return sqrt (dot (A, A));
}
double angle(Vector A, Vector B)    {       //向量转角，逆时针，点积
    return acos (dot (A, B) / length (A) / length (B));
}
Vector rotate(Vector A, double rad) {       //向量旋转，逆时针
    return Vector (A.x * cos (rad) - A.y * sin (rad), A.x * sin (rad) + A.y * cos (rad));
}
Vector nomal(Vector A)  {       //向量的单位法向量
    double len = length (A);
    return Vector (-A.y / len, A.x / len);
}
Point line_line_inter(Point p, Vector V, Point q, Vector W)    {        //两直线交点，参数方程
    Vector U = p - q;
    double t = cross (W, U) / cross (V, W);
    return p + V * t;
}
double point_to_line(Point p, Point a, Point b)   {       //点到直线的距离，两点式
    Vector V1 = b - a, V2 = p - a;
    return fabs (cross (V1, V2)) / length (V1);
}
double point_to_seg(Point p, Point a, Point b)    {       //点到线段的距离，两点式
    if (a == b) return length (p - a);
    Vector V1 = b - a, V2 = p - a, V3 = p - b;
    if (dcmp (dot (V1, V2)) < 0)    return length (V2);
    else if (dcmp (dot (V1, V3)) > 0)   return length (V3);
    else    return fabs (cross (V1, V2)) / length (V1);
}
Point point_line_proj(Point p, Point a, Point b)   {     //点在直线上的投影，两点式
    Vector V = b - a;
    return a + V * (dot (V, p - a) / dot (V, V));
}
bool can_seg_seg_inter(Point a1, Point a2, Point b1, Point b2)  {       //判断线段相交，两点式
    double c1 = cross (a2 - a1, b1 - a1), c2 = cross (a2 - a1, b2 - a1),
           c3 = cross (b2 - b1, a1 - b1), c4 = cross (b2 - b1, a2 - b1);
    return dcmp (c1) * dcmp (c2) < 0 && dcmp (c3) * dcmp (c4) < 0;
}
bool can_line_seg_inter(Point a1, Point a2, Point b1, Point b2)    {        //判断直线与线段相交，两点式
    double c1 = cross (a2 - a1, b1 - a1), c2 = cross (a2 - a1, b2 - a1);
    return dcmp (c1 * c2) <= 0;
}
bool on_seg(Point p, Point a1, Point a2)    {       //判断点在线段上，两点式
    return dcmp (cross (a1 - p, a2 - p)) == 0 && dcmp (dot (a1 - p, a2 - p)) < 0;
}

struct Line2 {
    double x1, x2;
    Line2 () {}
    Line2 (double x1, double x2) : x1 (x1), x2 (x2)  {}
    bool operator < (const Line2 &r) const   {
        return x1 < r.x1;
    }
};

struct Line    {
    Point a, b;
    Line () {}
    Line (Point a, Point b) : a (a), b (b) {}
};

int main(void)    {
    Line h, p;
    double x1, x2, y;
    while (scanf ("%lf%lf%lf", &x1, &x2, &y) == 3) {
        if (dcmp (x1) == 0 && dcmp (x2) == 0 && dcmp (y) == 0)    break;
        h.a = Point (x1, y);    h.b = Point (x2, y);
        scanf ("%lf%lf%lf", &x1, &x2, &y);
        p.a = Point (x1, y);    p.b = Point (x2, y);
        int n;  scanf ("%d", &n);
        vector<Line2> xs;
        for (int i=0; i<n; ++i) {
            scanf ("%lf%lf%lf", &x1, &x2, &y);
            if (dcmp (y - h.a.y) >= 0 || dcmp (y - p.a.y) <= 0) continue;
            Point p1 = Point (x1, y), p2 = Point (x2, y);
            x1 = line_line_inter (h.b, h.b - p1, p.b, p.b - p.a).x;
            x2 = line_line_inter (h.a, h.a - p2, p.b, p.b - p.a).x;
            if (dcmp (x1 - x2) >= 0)    continue;
            xs.push_back (Line2 (x1, x2));
        }
        xs.push_back (Line2 (p.b.x, p.b.x));
        sort (xs.begin (), xs.end ());
        double left = p.a.x, ans = 0;
        for (int i=0; i<xs.size (); ++i)    {
            x1 = xs[i].x1;  x2 = xs[i].x2;
            if (x1 - left > ans)   ans = x1 - left;
            if (dcmp (x2 - left) > 0)   {
                left = x2;
                if (dcmp (left - p.b.x) > 0)   break;
            }
        }
        if (dcmp (ans) == 0)    puts ("No View");
        else    printf ("%.2f\n", ans);
    }

    //cout << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";

    return 0;
}