240930 다각형의 면적

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

	int numberOfPoints = Integer.parseInt(br.readLine());
	long areaSum = 0;
	long[][] points = new long[numberOfPoints][2];

	for (int i = 0; i < numberOfPoints; i++) {
		st = new StringTokenizer(br.readLine());
		long x = Long.parseLong(st.nextToken());
		long y = Long.parseLong(st.nextToken());
		points[i][0] = x;
		points[i][1] = y;
	}

	long x1 = points[0][0];
	long y1 = points[0][1];
	for (int i = 1; i < numberOfPoints - 1; i++) {
		long x2 = points[i][0];
		long y2 = points[i][1];
		long x3 = points[i + 1][0];
		long y3 = points[i + 1][1];
		long tmp = x1 * y2 + x2 * y3 + x3 * y1 - x2 * y1 - x3 * y2 - x1 * y3;
		areaSum += tmp;
	}

	long absoluteArea = Math.abs(areaSum);
	if (absoluteArea % 2 == 0) {
		bw.write((absoluteArea / 2) + ".0\n");
	} else {
		bw.write((absoluteArea / 2) + ".5\n");
	}

	bw.flush();
	bw.close();
	br.close();
}

출처:https://www.acmicpc.net/problem/2166