跟前一篇3335的是一样的。
代码入下：
1 #include <cstdio> 2 #include <cstring> 3 #include <iostream> 4 #include <algorithm> 5 #include <vector> 6 #include <cmath> 7 8 using namespace std; 9 10 struct Point { 11 double x, y; 12 Point() {} 13 Point(double x, double y) : x(x), y(y) {} 14 } ; 15 template<class T> T sqr(T x) { return x * x;} 16 typedef Point Vec; 17 Vec operator + (Vec a, Vec b) { return Vec(a.x + b.x, a.y + b.y);} 18 Vec operator - (Vec a, Vec b) { return Vec(a.x - b.x, a.y - b.y);} 19 Vec operator * (Vec a, double p) { return Vec(a.x * p, a.y * p);} 20 Vec operator / (Vec a, double p) { return Vec(a.x / p, a.y / p);} 21 22 const double EPS = 1e-8; 23 const double PI = acos(-1.0); 24 inline int sgn(double x) { return (x > EPS) - (x < -EPS);} 25 26 inline double dotDet(Vec a, Vec b) { return a.x * b.x + a.y * b.y;} 27 inline double crossDet(Vec a, Vec b) { return a.x * b.y - a.y * b.x;} 28 inline double dotDet(Point o, Point a, Point b) { return dotDet(a - o, b - o);} 29 inline double crossDet(Point o, Point a, Point b) { return crossDet(a - o, b - o);} 30 inline double vecLen(Vec x) { return sqrt(dotDet(x, x));} 31 inline double toRad(double deg) { return deg / 180.0 * PI;} 32 inline double angle(Vec v) { return atan2(v.y, v.x);} 33 inline Vec vecUnit(Vec x) { return x / vecLen(x);} 34 inline Vec normal(Vec x) { return Vec(-x.y, x.x) / vecLen(x);} 35 36 const int N = 111; 37 struct DLine { 38 Point p; 39 Vec v; 40 double ang; 41 DLine() {} 42 DLine(Point p, Vec v) : p(p), v(v) { ang = atan2(v.y, v.x);} 43 bool operator < (DLine L) const { return ang < L.ang;} 44 DLine move(double x) { 45 Vec nor = normal(v); 46 nor = nor * x; 47 return DLine(p + nor, v); 48 } 49 } dl[N]; 50 Point pt[N]; 51 52 inline bool onLeft(DLine L, Point p) { return crossDet(L.v, p - L.p) > 0;} 53 Point dLineIntersect(DLine a, DLine b) { 54 Vec u = a.p - b.p; 55 double t = crossDet(b.v, u) / crossDet(a.v, b.v); 56 return a.p + a.v * t; 57 } 58 59 struct Poly { 60 vector<Point> pt; 61 Poly() { pt.clear();} 62 ~Poly() {} 63 Poly(vector<Point> &pt) : pt(pt) {} 64 Point operator [] (int x) { return pt[x];} 65 int size() { return pt.size();} 66 double area() { 67 double ret = 0.0; 68 int sz = pt.size(); 69 pt.push_back(pt[0]); 70 for (int i = 1; i <= sz; i++) ret += crossDet(pt[i], pt[i - 1]); 71 pt.pop_back(); 72 return fabs(ret / 2.0); 73 } 74 } ; 75 76 Poly halfPlane(DLine *L, int n) { 77 Poly ret = Poly(); 78 sort(L, L + n); 79 int fi, la; 80 Point *p = new Point[n]; 81 DLine *q = new DLine[n]; 82 q[fi = la = 0] = L[0]; 83 for (int i = 1; i < n; i++) { 84 while (fi < la && !onLeft(L[i], p[la - 1])) la--; 85 while (fi < la && !onLeft(L[i], p[fi])) fi++; 86 q[++la] = L[i]; 87 if (sgn(crossDet(q[la].v, q[la - 1].v)) == 0) { 88 la--; 89 if (onLeft(q[la], L[i].p)) q[la] = L[i]; 90 } 91 if (fi < la) p[la - 1] = dLineIntersect(q[la - 1], q[la]); 92 } 93 while (fi < la && !onLeft(q[fi], p[la - 1])) la--; 94 if (la <= fi) return ret; 95 p[la] = dLineIntersect(q[la], q[fi]); 96 for (int i = fi; i <= la; i++) ret.pt.push_back(p[i]); 97 return ret; 98 } 99 100 bool isClockwise(Point *pt, int n) { 101 double sum = 0.0; 102 pt[n] = pt[0]; 103 Point O = Point(0.0, 0.0); 104 for (int i = 0; i < n; i++) { 105 sum += crossDet(O, pt[i], pt[i + 1]); 106 } 107 return sum < 0; 108 } 109 110 int main() { 111 // freopen("in", "r", stdin); 112 int T = 1, n; 113 while (cin >> n && n) { 114 for (int i = 0; i < n; i++) cin >> pt[i].x >> pt[i].y; 115 pt[n] = pt[0]; 116 if (isClockwise(pt, n)) for (int i = 0; i < n; i++) dl[i] = DLine(pt[i + 1], pt[i] - pt[i + 1]).move(-EPS); 117 else for (int i = 0; i < n; i++) dl[i] = DLine(pt[i], pt[i + 1] - pt[i]).move(-EPS); 118 Poly tmp = halfPlane(dl, n); 119 printf("Floor #%d Surveillance is ", T++); 120 if (tmp.size() > 2) puts("possible."); 121 else puts("impossible."); 122 puts(""); 123 } 124 return 0; 125 } 126 127 /* 128 6 129 0 0 130 100 0 131 100 100 132 0 100 133 50 75 134 50 25 135 4 136 0 0 137 0 1 138 1 1 139 1 0 140 8 141 0 0 142 0 2 143 1 2 144 1 1 145 2 1 146 2 2 147 3 2 148 3 0 149 */
——written by Lyon