241031 로버트 후드

static class Point {
	int x;
	int y;
	Point(int x, int y) { this.x = x; this.y = y; }
}

static class PointComparator implements Comparator<Point> {
	@Override
	public int compare(Point a, Point b) {
		return a.x == b.x ? a.y - b.y : a.x - b.x;
	}
}

public static void main(String[] args) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	int numPoints = Integer.parseInt(br.readLine());

	Point[] points = new Point[numPoints];
	int[] prev = new int[numPoints];
	int[] next = new int[numPoints];

	for (int i = 0; i < numPoints; i++) {
		StringTokenizer st = new StringTokenizer(br.readLine());
		int x = Integer.parseInt(st.nextToken());
		int y = Integer.parseInt(st.nextToken());
		points[i] = new Point(x, y);
		prev[i] = next[i] = -1;
	}

	Arrays.sort(points, 0, numPoints, new PointComparator());
	initializeConvexHull(prev, next, points, numPoints);

	int maxDistance = findMaxDistance(points, next);
	System.out.printf("%.10f", Math.sqrt(maxDistance));
}

private static void initializeConvexHull(int[] prev, int[] next, Point[] points, int numPoints) {
	prev[0] = next[0] = 1;
	prev[1] = next[1] = 0;
	
	for (int i = 2; i < numPoints; i++) {
		int top = i - 1;
		int bottom = i - 1;
		while (top > 0 && ccw(points[i], points[top], points[next[top]]) < 1) {
			top = next[top];
		}
		while (bottom > 0 && ccw(points[i], points[bottom], points[prev[bottom]]) > -1) {
			bottom = prev[bottom];
		}
		prev[top] = i;
		next[i] = top;
		next[bottom] = i;
		prev[i] = bottom;
	}
}

private static int findMaxDistance(Point[] points, int[] next) {
	int maxDistance = 0;
	int start = 0;
	int end = next[start];
	maxDistance = distanceSquared(points[start], points[end]);

	while (end != 0) {
		while (end != start) {
			maxDistance = Math.max(maxDistance, distanceSquared(points[start], points[end]));
			if (ccw(new Point(0, 0), subtract(points[next[start]], points[start]), subtract(points[next[end]], points[end])) < 1) {
				break;
			}
			end = next[end];
		}
		start = next[start];
		if (start == 0) break;
	}
	return maxDistance;
}

private static int ccw(Point p1, Point p2, Point p3) {
	int value = (p1.x - p2.x) * (p2.y - p3.y) - (p3.x - p2.x) * (p2.y - p1.y);
	return Integer.compare(value, 0);
}

private static int distanceSquared(Point p1, Point p2) {
	int dx = p1.x - p2.x;
	int dy = p1.y - p2.y;
	return dx * dx + dy * dy;
}

private static Point subtract(Point p1, Point p2) {
	return new Point(p1.x - p2.x, p1.y - p2.y);
}

출처:https://www.acmicpc.net/problem/9240