AOJ:0012

gistfile1.java

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		while (scn.hasNext()) {
			double x1 = scn.nextDouble();
			double y1 = scn.nextDouble();
			double[] xarr = new double[3];
			double[] yarr = new double[3];
			xarr[0] = scn.nextDouble() - x1;
			yarr[0] = scn.nextDouble() - y1;
			xarr[1] = scn.nextDouble() - x1;
			yarr[1] = scn.nextDouble() - y1;
			xarr[2] = scn.nextDouble() - x1;
			yarr[2] = scn.nextDouble() - y1;
			double[] sol = simultaneousEquation(xarr, yarr);
			if(sol[0] > 0 && sol[1] > 0 && sol[0] + sol[1] < 1) {
				System.out.println("YES");
			} else {
				System.out.println("NO");
			}
		}
	}
	public static double[] simultaneousEquation(double[] a, double[] b) {
		double[] arr = new double[2];
		if (a[0] * b[1] - a[1] * b[0] == 0) {
			arr[0] = Double.NaN;
			arr[1] = Double.NaN;
		} else {
			arr[0] = (b[1] * a[2] - a[1] * b[2]) / (a[0] * b[1] - a[1] * b[0]);
			arr[1] = (a[0] * b[2] - b[0] * a[2]) / (a[0] * b[1] - a[1] * b[0]);
		}
		return arr;
	}
}