POJ 2653 Pick-up sticks（计算几何，判断线段相交）

解题思路：

暴力循环，如果一个线段后面的所有线段都不和它相交，则这个线段是top的。

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#define LL long long
using namespace std;
const int MAXN = 100000 + 10;
struct Point
{
double x, y;
Point(double x = 0, double y = 0) : x(x), y(y) { }
};
typedef Point Vector;
const double eps = 1e-10;
int dcmp(double x)
{
if(fabs(x) < eps) return 0;
else return x < 0 ? -1 : 1;
}
Vector operator + (Vector A, Vector B) { return Vector(A.x + B.x, A.y + B.y); }
Vector operator - (Vector A, Vector B) { return Vector(A.x - B.x, A.y - B.y); }
Vector operator * (Vector A, double p) { return Vector(A.x * p, A.y * p); }
Vector operator / (Vector A, double p) { return Vector(A.x / p, A.y / p); }
bool operator < (const Point& a, const Point& b)
{
return a.x < b.x || (a.x == b.x && a.y < b.y);
}
bool operator == (const Point& a, const Point& b)
{
return dcmp(a.x - b.x) == 0 && dcmp(a.y - b.y) == 0;
}
double Cross(Vector A, Vector B)
{
return A.x * B.y - A.y * B.x;
}
bool SegmentIntersection(Point a1, Point a2, Point b1, Point b2)
{
double c1 = Cross(a2 - a1, b1 - a1), c2 = Cross(a2 - a1, b2 - a1);
double c3 = Cross(b2 - b1, a1 - b1), c4 = Cross(b2 - b1, a2 - b1);
return dcmp(c1) * dcmp(c2) < 0 && dcmp(c3) * dcmp(c4) < 0;
}
int A[MAXN];
struct Line
{
Point x, y;
}L[MAXN];
int main()
{
int N;
while(scanf("%d", &N)!=EOF)
{
if(N == 0) break;
for(int i=1;i<=N;i++)
{
scanf("%lf%lf%lf%lf", &L[i].x.x, &L[i].x.y, &L[i].y.x, &L[i].y.y);
}
int m = 0;
for(int i=1;i<=N;i++)
{
bool flag = true;
for(int j=i+1;j<=N;j++)
{
if(SegmentIntersection(L[i].x, L[i].y, L[j].x, L[j].y))
{
flag = false;
break;
}
}
if(flag) A[++m] = i;
}
printf("Top sticks: ");
for(int i=1;i<m;i++)
printf("%d, ", A[i]);
printf("%d.\n", A[m]);
}
return 0;
}