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You are given a list of airline tickets where tickets[i] = [from_i, to_i] represent the departure and the arrival airports of one flight. Reconstruct the itinerary in order and return it. All of the tickets belong to a man who departs from "JFK", thus, the itinerary must begin with "JFK". If there are multiple valid itineraries, you should return the itinerary that has the smallest lexical order when read as a single string. You may assume all tickets form at least one valid itinerary. You must use all the tickets once and only once.
Example 1
Input: tickets = [["MUC","LHR"],["JFK","MUC"],["SFO","SJC"],["LHR","SFO"]]
Output: ["JFK","MUC","LHR","SFO","SJC"]
Explanation: Starting at JFK, each airport has exactly one outgoing ticket: JFK → MUC → LHR → SFO → SJC. This uses all four tickets.
Example 2
Input: tickets = [["JFK","SFO"],["JFK","ATL"],["SFO","ATL"],["ATL","JFK"],["ATL","SFO"]]
Output: ["JFK","ATL","JFK","SFO","ATL","SFO"]
Explanation: From JFK we could go to ATL or SFO, and ATL comes first alphabetically. JFK → ATL → JFK → SFO → ATL → SFO uses every ticket once and is the alphabetically smallest valid route.
Example 3
Input: tickets = [["JFK","A"],["A","JFK"]]
Output: ["JFK","A","JFK"]
Explanation: Fly JFK → A, then A → JFK. Both tickets are used.
Using every ticket exactly once is an Eulerian path. Greedily walk to the smallest available destination; when stuck, that airport is appended to the route. Reversing the collected order yields the itinerary.
Run checks all cases above. Submit evaluates all test cases.