Grand final

Challenge.java

import java.util.*;
public class Challenge {
	public static void main(String[] args) throws Exception {
		// Build map.
		Map<String,Set<String>> graph = new HashMap<String,Set<String>>();
		for (String rel : Input.read().split("\\,")) {
			String[] toks = rel.split("\\-");
			Set<String> friends = graph.containsKey(toks[0]) ? graph.get(toks[0]) : new HashSet<String>();
			graph.put(toks[0], friends);
			friends.add(toks[1]);
			
			friends = graph.containsKey(toks[1]) ? graph.get(toks[1]) : new HashSet<String>();
			graph.put(toks[1], friends);
			friends.add(toks[0]);
		}
		
		Set<String> fofs = new HashSet<String>();
		
		// For everyone other than John
		for (Map.Entry<String,Set<String>> entry : graph.entrySet()) {
			if (entry.getKey().equals("1")) {
				continue;
			}
			Set<String> common = new HashSet<String>(graph.get("1"));
			common.retainAll(entry.getValue());
			if (common.size() == 1) {
				fofs.add(entry.getKey());
			}
		}
		List<String> fofList = new ArrayList<String>(fofs);
		Collections.sort(fofList);
		StringBuilder sb = new StringBuilder();
		for (String f : fofList) {
			sb.append(",").append(f);
		}
		Output.write(sb.substring(1));
	}
}