241002 선분 교차 1

public static void main(String[] args) throws IOException {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	StringTokenizer st = new StringTokenizer(br.readLine());

	long x1 = Long.parseLong(st.nextToken());
	long y1 = Long.parseLong(st.nextToken());

	long x2 = Long.parseLong(st.nextToken());
	long y2 = Long.parseLong(st.nextToken());

	st = new StringTokenizer(br.readLine());
	long x3 = Long.parseLong(st.nextToken());
	long y3 = Long.parseLong(st.nextToken());

	long x4 = Long.parseLong(st.nextToken());
	long y4 = Long.parseLong(st.nextToken());

	System.out.println(computeIntersection(x1, y1, x2, y2, x3, y3, x4, y4));
	br.close();
}

private static int computeIntersection(long x1, long y1, long x2, long y2, long x3, long y3, long x4, long y4) {
	int ccw1 = computeCCW(x1, y1, x2, y2, x3, y3);
	int ccw2 = computeCCW(x1, y1, x2, y2, x4, y4);
	int ccw3 = computeCCW(x3, y3, x4, y4, x1, y1);
	int ccw4 = computeCCW(x3, y3, x4, y4, x2, y2);

	if (ccw1 * ccw2 < 0 && ccw3 * ccw4 < 0) {
		return 1;
	}
	return 0;
}

private static int computeCCW(long x1, long y1, long x2, long y2, long x3, long y3) {
	long crossProduct = (x2 * y3 + x3 * y1 + x1 * y2) - (y1 * x2 + y2 * x3 + y3 * x1);
	if (crossProduct > 0) {
		return 1;
	}
	return -1;
}

출처:https://www.acmicpc.net/problem/17386