http://www.reddit.com/r/dailyprogrammer/comments/1iambu/071513_challenge_133_easy_foottraffic_analysis/

FootTrafficAnalysis.java

import java.util.*;
import java.io.*;

public class FootTrafficAnalysis {
	
	HashMap<Integer, HashMap<Integer, ArrayList<Integer>>> dataStore = 
		new HashMap<Integer, HashMap<Integer, ArrayList<Integer>>>();

	public static void main(String[] args) {
		new FootTrafficAnalysis().channelLogFile(new File("simple.in"));	
		new FootTrafficAnalysis().channelLogFile(new File("simple2.in"));
	}

	public void channelLogFile(File file) {
		Scanner scanner = null;

		try {
			scanner = new Scanner(file);
		} catch (FileNotFoundException ex) {
			System.err.println(ex);
		}
		int N = Integer.parseInt(scanner.nextLine());
		while(scanner.hasNextLine()) {
			String log = scanner.nextLine();
			processLog(log);
		}

		for(Integer room : dataStore.keySet()) {
			HashMap<Integer, ArrayList<Integer>> visitors = dataStore.get(room);
			int total = 0;
			int totalVisitors = 0;
			for(Integer visitor : visitors.keySet()) {
				totalVisitors += 1;
				ArrayList<Integer> times = visitors.get(visitor);
				Collections.sort(times);
				int len = times.size();
				if(times.size() % 2 != 0) {
					throw new RuntimeException("visitor does not leave the room");
				}
				
				for(int i = 0; i < len; i += 2) {
					total += times.get(i + 1) - times.get(i);
				}
			}
			int average = total / totalVisitors;
			System.out.println(
				"Room " 
				+ room 
				+ ", " 
				+ average 
				+ " minute average visit, " 
				+ totalVisitors 
				+ " visitor total");
		}
	}

	private void processLog(String log) {
		TrafficLog trafficLog = TrafficLog.getInstance(log);
		int room = trafficLog.getRoomId();
		int visitor = trafficLog.getVisitorId();
		int time = trafficLog.getTimeStamp();

		HashMap<Integer, ArrayList<Integer>> roomData = null;
		if(dataStore.containsKey(room)) {
			roomData = dataStore.get(room);
		} else {
			roomData = new HashMap<Integer, ArrayList<Integer>>();
			dataStore.put(room, roomData);
		}
		
		ArrayList<Integer> times = null;
		if(roomData.containsKey(visitor)) {
			times = roomData.get(visitor);
		} else {
			times = new ArrayList<Integer>();
			roomData.put(visitor, times);
		}

		times.add(time);

	}
}



class TrafficLog {
	public static char EnterEvent = 'I';
	public static char ExitEvent = 'O';
	private int visitorId;
	private int roomId;
	private char eventType;
	private int timeStamp;

	public static TrafficLog getInstance(String log) {
		String[] logArray = log.split(" ");
		if(logArray.length < 4) throw new RuntimeException("malformed log.");
		return new TrafficLog(
			Integer.parseInt(logArray[0]),
			Integer.parseInt(logArray[1]),
			logArray[2].charAt(0),
			Integer.parseInt(logArray[3])
		);
	}

	private TrafficLog(int visitorId, int roomId, char eventType, int time) {
		if(eventType != EnterEvent && eventType != ExitEvent) {
			throw new RuntimeException("unknown event type.");
		}
		this.visitorId = visitorId;
		this.roomId = roomId;
		this.eventType = eventType;
		this.timeStamp = time;
	}

	public int getRoomId() {
		return this.roomId;
	}

	public int getVisitorId() {
		return this.visitorId;
	}

	public char getEventType() {
		return this.eventType;
	}

	public int getTimeStamp() {
		return this.timeStamp;
	}
}

test1.in

4
0 0 I 540
1 0 I 540
0 0 O 560
1 0 O 560

test1.out

Room 0, 20 minute average visit, 2 visitor total

test2.in

36
0 11 I 347
1 13 I 307
2 15 I 334
3 6 I 334
4 9 I 334
5 2 I 334
6 2 I 334
7 11 I 334
8 1 I 334
0 11 O 376
1 13 O 321
2 15 O 389
3 6 O 412
4 9 O 418
5 2 O 414
6 2 O 349
7 11 O 418
8 1 O 418
0 12 I 437
1 28 I 343
2 32 I 408
3 15 I 458
4 18 I 424
5 26 I 442
6 7 I 435
7 19 I 456
8 19 I 450
0 12 O 455
1 28 O 374
2 32 O 495
3 15 O 462
4 18 O 500
5 26 O 479
6 7 O 493
7 19 O 471
8 19 O 458

test2.out

Room 1, 85 minute average visit, 1 visitor total
Room 11, 57 minute average visit, 2 visitors total
Room 12, 19 minute average visit, 1 visitor total
Room 13, 15 minute average visit, 1 visitor total
Room 15, 30 minute average visit, 2 visitors total
Room 18, 77 minute average visit, 1 visitor total
Room 19, 12 minute average visit, 2 visitors total
Room 2, 48 minute average visit, 2 visitors total
Room 26, 38 minute average visit, 1 visitor total
Room 28, 32 minute average visit, 1 visitor total
Room 32, 88 minute average visit, 1 visitor total
Room 6, 79 minute average visit, 1 visitor total
Room 7, 59 minute average visit, 1 visitor total
Room 9, 85 minute average visit, 1 visitor total