LA 2797 (平面直线图PLSG) Monster Trap

  1 //#define LOCAL
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <algorithm>
  5 #include <iostream>
  6 #include <cmath>
  7 #include <vector>
  8 using namespace std;
  9 
 10 const double eps = 1e-12;
 11 int dcmp(double x)
 12 {
 13     if(fabs(x) < eps) return 0;
 14     else return x < 0 ? -1 : 1;
 15 }
 16 
 17 struct Point
 18 {
 19     double x, y;
 20     Point(double x=0, double y=0):x(x), y(y) {}
 21 };
 22 typedef Point Vector;
 23 
 24 Point operator + (const Point& a, const Point& b) { return Point(a.x+b.x, a.y+b.y); }
 25 Point operator - (const Point& a, const Point& b) { return Point(a.x-b.x, a.y-b.y); }
 26 Vector operator * (const Vector& a, double p) { return Vector(a.x*p, a.y*p); }
 27 Vector operator / (const Vector& a, double p) { return Vector(a.x/p, a.y/p); }
 28 bool operator < (const Point& a, const Point& b)
 29 {
 30     return a.x < b.x || (a.x == b.x && a.y < b.y);
 31 }
 32 bool operator == (const Point& a, const Point& b)
 33 {
 34     return a.x == b.x && a.y == b.y;
 35 }
 36 double Dot(const Vector& a, const Vector& b) { return a.x*b.x + a.y*b.y; }
 37 double Cross(const Vector& a, const Vector& b) { return a.x*b.y - a.y*b.x; }
 38 double Length(const Vector& a) { return sqrt(Dot(a, a)); }
 39 bool SegmentProperIntersection(const Point& a1, const Point& a2, const Point& b1, const Point& b2)
 40 {
 41     double c1 = Cross(a2-a1, b1-a1), c2 = Cross(a2-a1, b2-a1);
 42     double c3 = Cross(b2-b1, a1-b1), c4 = Cross(b2-b1, a2-b1);
 43     return dcmp(c1)*dcmp(c2) < 0 && dcmp(c3)*dcmp(c4) < 0;
 44 }
 45 
 46 bool OnSegment(const Point& p, const Point& a, const Point& b)
 47 {
 48     return dcmp(Cross(a-p, b-p)) == 0 && dcmp(Dot(a-p, b-p)) < 0;
 49 }
 50 
 51 const int maxn = 200 + 5;
 52 int n, V;
 53 int G[maxn][maxn], vis[maxn];
 54 Point p1[maxn], p2[maxn];
 55 
 56 bool OnAnySegment(const Point& p)
 57 {
 58     for(int i = 0; i < n; ++i)
 59         if(OnSegment(p, p1[i], p2[i]))    return true;
 60     return false;
 61 }
 62 
 63 bool IntersectionWithAnySegment(const Point& a, const Point& b)
 64 {
 65     for(int i = 0; i < n; ++i)
 66         if(SegmentProperIntersection(a, b, p1[i], p2[i]))    return true;
 67     return false;
 68 }
 69 
 70 bool dfs(int u)
 71 {
 72     if(u == 1)    return true;
 73     vis[u] = 1;
 74     for(int v = 0; v < V; ++v)
 75         if(G[u][v] && !vis[v] && dfs(v))    return true;
 76     return false;
 77 }
 78 
 79 bool find_path()
 80 {
 81     vector<Point> vertices;
 82     vertices.push_back(Point(0.0, 0.0));
 83     vertices.push_back(Point(1e5, 1e5));
 84     for(int i = 0; i < n; ++i)
 85     {
 86         if(!OnAnySegment(p1[i])) vertices.push_back(p1[i]);
 87         if(!OnAnySegment(p2[i])) vertices.push_back(p2[i]);
 88     }
 89     V = vertices.size();
 90     memset(G, 0, sizeof(G));
 91     memset(vis, 0, sizeof(vis));
 92     for(int i = 0; i < V; ++i)
 93         for(int j = i+1; j < V; ++j)
 94             if(!IntersectionWithAnySegment(vertices[i], vertices[j]))
 95                 G[i][j] = G[j][i] = 1;
 96     return dfs(0);
 97 }
 98 
 99 int main(void)
100 {
101     #ifdef LOCAL
102         freopen("2797in.txt", "r", stdin);
103     #endif
104     
105     while(scanf("%d", &n) == 1 && n)
106     {
107         double x1, y1, x2, y2;
108         for(int i = 0; i < n; ++i)
109         {
110             scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2);
111             Point a = Point(x1, y1);
112             Point b = Point(x2, y2);
113             double l = Length(a-b);
114             Vector v0 = (a-b) / l * 1e-6;
115             p1[i] = a + v0;
116             p2[i] = b - v0;
117         }
118         if(find_path()) puts("no");
119         else puts("yes");
120     }
121     
122     return 0;
123 }