链接：
http://poj.org/userstatus?user_id=Flowersea
题意：
给你n条线段，问你有多少条线段不被后面的线段压着
题解：
数据比较水，直接n²
代码：
1 #include <map> 2 #include <set> 3 #include <cmath> 4 #include <queue> 5 #include <stack> 6 #include <cstdio> 7 #include <string> 8 #include <vector> 9 #include <cstdlib> 10 #include <cstring> 11 #include <sstream> 12 #include <iostream> 13 #include <algorithm> 14 #include <functional> 15 using namespace std; 16 #define rep(i,a,n) for (int i=a;i<n;i++) 17 #define per(i,a,n) for (int i=n-1;i>=a;i--) 18 #define all(x) (x).begin(),(x).end() 19 #define pb push_back 20 #define mp make_pair 21 #define lson l,m,rt<<1 22 #define rson m+1,r,rt<<1|1 23 typedef long long ll; 24 typedef vector<int> VI; 25 typedef pair<int, int> PII; 26 const ll MOD = 1e9 + 7; 27 const int INF = 0x3f3f3f3f; 28 const int MAXN = 1e5 + 7; 29 // head 30 31 const double eps = 1e-8; 32 int cmp(double x) { 33 if (fabs(x) < eps) return 0; 34 if (x > 0) return 1; 35 return -1; 36 } 37 38 const double pi = acos(-1); 39 inline double sqr(double x) { 40 return x*x; 41 } 42 struct point { 43 double x, y; 44 point() {} 45 point(double a, double b) :x(a), y(b) {} 46 void input() { 47 scanf("%lf%lf", &x, &y); 48 } 49 friend point operator+(const point &a, const point &b) { 50 return point(a.x + b.x, a.y + b.y); 51 } 52 friend point operator-(const point &a, const point &b) { 53 return point(a.x - b.x, a.y - b.y); 54 } 55 friend point operator*(const double &a, const point &b) { 56 return point(a*b.x, a*b.y); 57 } 58 friend point operator/(const point &a, const double &b) { 59 return point(a.x / b, a.y / b); 60 } 61 double norm() { 62 return sqrt(sqr(x) + sqr(y)); 63 } 64 }; 65 double det(point a, point b) { 66 return a.x*b.y - a.y*b.x; 67 } 68 double dot(point a, point b) { 69 return a.x*b.x + a.y*b.y; 70 } 71 72 struct line { 73 point a, b; 74 line() {} 75 line(point x, point y) :a(x), b(y) {} 76 }p[MAXN]; 77 bool point_on_segment(point p, point s, point t) { 78 return cmp(det(p - s, t - s)) == 0 && cmp(dot(p - s, p - t)) <= 0; 79 } 80 bool parallel(line a, line b) { 81 return !cmp(det(a.a - a.b, b.a - b.b)); 82 } 83 bool line_make_point(line a, line b,point &res) { 84 if (parallel(a, b)) return false; 85 double s1 = det(a.a - b.a, b.b - b.a); 86 double s2 = det(a.b - b.a, b.b - b.a); 87 res = (s1*a.b - s2*a.a) / (s1 - s2); 88 return true; 89 } 90 91 int main() { 92 int n; 93 while (cin >> n, n) { 94 rep(i, 1, n + 1) { 95 point a, b; 96 a.input(), b.input(); 97 p[i] = line(a, b); 98 } 99 VI ans; 100 rep(i, 1, n) { 101 int fg = 1; 102 rep(j, i + 1, n + 1) { 103 point res; 104 if (line_make_point(p[i], p[j], res)) 105 if (point_on_segment(res, p[i].a, p[i].b) && 106 point_on_segment(res, p[j].a, p[j].b)) { 107 fg = 0; 108 break; 109 } 110 } 111 if (fg) ans.pb(i); 112 } 113 cout << "Top sticks: "; 114 rep(i, 0, ans.size()) cout << ans[i] << ", "; 115 cout << n << '.' << endl; 116 } 117 return 0; 118 }