0332-reconstruct-itinerary.java

class Solution {

    public List<String> findItinerary(List<List<String>> tickets) {
        LinkedList<String> itinerary = new LinkedList<>();
        Map<String, PriorityQueue<String>> graph = new HashMap<>();
        Stack<String> stack = new Stack<>();

        for (List<String> ticket : tickets) {
            graph
                .computeIfAbsent(ticket.get(0), k -> new PriorityQueue<>())
                .add(ticket.get(1));
        }

        stack.push("JFK");
        while (!stack.isEmpty()) {
            String nextDestination = stack.peek();

            if (
                !graph
                    .getOrDefault(nextDestination, new PriorityQueue<>())
                    .isEmpty()
            ) {
                stack.push(graph.get(nextDestination).poll());
            } else {
                itinerary.addFirst(stack.pop());
            }
        }
        return itinerary;
    }
}