POJ 1127 Jack Straws ( 求直线交点， 判断线段是否相交（含端点） )

#include <iostream>
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <queue>
#include <map>
#include <vector>
#include <set>
#include <string>
#include <math.h>
#define LL long long
#define mem(i, j) memset(i, j, sizeof(i))
#define rep(i, j, k) for(int i = j; i <= k; i++)
#define dep(i, j, k) for(int i = k; i >= j; i--)
#define pb push_back
#define make make_pair
#define INF INT_MAX
#define inf LLONG_MAX
#define PI acos(-1)
using namespace std;

const int N = 310;

struct Point {
    double x, y;
    Point(double x = 0, double y = 0) : x(x), y(y) { } /// 构造函数
};

typedef Point Vector;
/// 向量+向量=向量， 点+向量=向量
Vector operator + (Vector A, Vector B) { return Vector(A.x + B.x, A.y + B.y); }
///点-点=向量
Vector operator - (Point A, Point B) { return Vector(A.x - B.x, A.y - B.y); }
///向量*数=向量
Vector operator * (Vector A, double p) { return Vector(A.x * p, A.y * p); }
///向量/数=向量
Vector operator / (Vector A, double p) { return Vector(A.x / p, A.y / p); }

const double eps = 1e-10;
int dcmp(double x) {
    if(fabs(x) < eps) return 0; else return x < 0 ? -1 : 1;
}

bool operator < (const Point& a, const Point& b) {
    return a.x == b.x ? a.y < b.y : a.x < b.x;
}

bool operator == (const Point& a, const Point &b) {
    return dcmp(a.x - b.x) == 0 && dcmp(a.y - b.y) == 0;
}

double Dot(Vector A, Vector B) { return A.x*B.x + A.y*B.y; } /// 点积
double Length(Vector A) { return sqrt(Dot(A, A)); } /// 计算向量长度
double Angle(Vector A, Vector B) { return acos(Dot(A, B) / Length(A) / Length(B)); } /// 向量A、B夹角
double Cross(Vector A, Vector B) { return A.x*B.y - A.y*B.x; } /// 叉积

Point GetLineIntersection(Point P, Vector v, Point Q, Vector w) { /// 求两直线交点，请确保 P + tv 和 Q + tw 有唯一交点。 当且仅当Cross(v, w)非0, v,w为直线的方向向量， P， Q为直线上任一点
    Vector u = P - Q;
    double t = Cross(w, u) / Cross(v, w);
    return P + v * t;
}

bool OnSegment(Point p, Point a1, Point a2) { /// 点p是否在线段a1,a2上（包含端点）
    return dcmp(Cross(a1 - p, a2 - p)) == 0 && dcmp(Dot(a1 - p, a2 - p)) <= 0;
}

bool SegmentProperInsection(Point a1, Point a2, Point b1, Point b2) {/// 判断线段是否相交,包括端点
    if(dcmp(Cross(a1 - a2, b1 - b2)) == 0)
        return OnSegment(b1, a1, a2) || OnSegment(b2, a1, a2) || OnSegment(a1, b1, b2) || OnSegment(a2, b1, b2);
    else {
        Point t = GetLineIntersection(a1, a2 - a1, b1, b2 - b1);
        return OnSegment(t, a1, a2) && OnSegment(t, b1, b2);
    }
}

Point P[100], Q[100];
bool ans[100][100];
int main() {
    int n;
    while(scanf("%d", &n) == 1 && n ) {
        rep(i, 1, n) {
            scanf("%lf %lf %lf %lf", &P[i].x, &P[i].y, &Q[i].x, &Q[i].y);
        }
        mem(ans, 0);
        rep(i, 1, n) {
            ans[i][i] = 1;
            rep(j, 1, i - 1) {
                ans[i][j] = ans[j][i] = SegmentProperInsection(P[i], Q[i], P[j], Q[j]);
            }
        }
        rep(i, 1, n) rep(j, 1, n) rep(k, 1, n) ans[j][k] |= ans[j][i] && ans[i][k];
        int x, y;
        while(scanf("%d %d", &x, &y)) {
            if(x + y == 0) break;
            if(ans[x][y]) printf("CONNECTED\n");
            else printf("NOT CONNECTED\n");
        }
    }
    return 0;
}