LeetCode – Reconstruct Itinerary (Java)

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Given a list of airline tickets represented by pairs of departure and arrival airports [from, to], reconstruct the itinerary in order. All of the tickets belong to a man who departs from JFK. Thus, the itinerary must begin with JFK.

Analysis

This is an application of Hierholzer’s algorithm to find a Eulerian path.

PriorityQueue should be used instead of TreeSet, because there are duplicate entries.

Java Solution

public class Solution{
	HashMap<String, PriorityQueue<String>> map = new HashMap<String, PriorityQueue<String>>();
	LinkedList<String> result = new LinkedList<String>();
 
	public List<String> findItinerary(String[][] tickets) {
		for (String[] ticket : tickets) {
			if (!map.containsKey(ticket[0])) {
				PriorityQueue<String> q = new PriorityQueue<String>();
				map.put(ticket[0], q);
			}
			map.get(ticket[0]).offer(ticket[1]);
		}
 
		dfs("JFK");
		return result;
	}
 
	public void dfs(String s) {
		PriorityQueue<String> q = map.get(s);
 
		while (q != null && !q.isEmpty()) {
			dfs(q.poll());
		}
 
		result.addFirst(s);
	}
}

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