Flight Route Planner

You are given a collection of airline flight segments represented as pairs [origin, destination], where each pair indicates a direct flight from the origin airport to the destination airport. Your task is to construct a complete travel itinerary that uses every flight segment exactly once.

All flight segments belong to a single traveler whose journey must begin at the airport with code "JFK". Therefore, your reconstructed itinerary must start from "JFK".

When multiple valid itineraries exist (i.e., multiple ways to use all the flights), you must return the itinerary that is lexicographically smallest when read as a sequence of airport codes. In other words, among all possible valid routes, prefer the one where, at each step, you choose the alphabetically earliest available destination.

Key Observations:

• It is guaranteed that all given flights can form at least one valid itinerary
• Each flight segment must be used exactly once—no more, no less
• The resulting itinerary will contain n + 1 airport codes, where n is the number of flight segments
• The same airport code may appear multiple times in the itinerary if you fly through it more than once