Reconstruct route from unordered travel tickets

You are given a list of directed travel tickets, each represented as a pair `[from, to]` (city names are strings), e.g. `[["A","B"],["B","C"], ...]`. The tickets form exactly one valid trip that visits cities in a single chain (no branching): - Every city has **at most one outgoing** ticket and **at most one incoming** ticket. - All tickets belong to the same trip. - The starting city is **not given**. Return the full ordered itinerary as a list of cities, e.g. given `[["A","B"],["B","C"]]`, return `["A","B","C"]`. **Constraints** - `1 <= tickets.length <= 2e5` - City names are non-empty strings. - The input is guaranteed to form one valid chain. **Output** - A list of cities of length `tickets.length + 1` representing the reconstructed route.