Solution.java

import java.io.*;
import java.util.*;
import java.util.stream.*;
import com.google.common.collect.ImmutableMap;

/*
Given a list of flights (in any order), construct the trip that this list represents. For example, if we have a flight from San Francisco to Los Angeles and a flight from New York City to San Francisco, the trip is "NYC to SFO to LAX".
Assumptions:
- A city will only be visited once per trip. (you can't go back to a given city after visiting it).
- The list will only represent one single trip.

# Flights:
# ORD -> DNV
# SFO -> NYC
# LAX -> SFO
# NYC -> ORD
# 
# Trip:
# LAX -> SFO -> NYC -> ORD -> DNV


flights = [("ORD", "DNV"), ("SFO", "NYC"), ("LAX", "SFO"), ("NYC", "ORD")]
*/
class Solution {
  
  static Map<String,String> flights = ImmutableMap.of("ORD","DNV","SFO","NYC","LAX","SFO","NYC","ORD");
    
  public static void main(String[] args) {

    String starting = flights.keySet().stream()
        .filter(x -> !flights.values().contains(x))
        .findFirst().get();
    String ending = flights.values().stream()
        .filter(x -> !flights.keySet().contains(x))
        .findFirst().get();
    
    List<String> trip = new ArrayList<String>();
    trip.add(starting);

    String dest = flights.get(starting);
    while (true) {

      trip.add(dest);
      
      if (dest.equals(ending))
        break;
      
      dest = flights.get(dest);
      
    }

    System.out.println(trip.stream().collect(Collectors.joining(" -> ")));
    
  }
  
}