My initial take on a itinerary problem

Flights.swift

struct Flight {
    let fromCity: String
    let toCity: String
}

let originCities = itinerary1.map { $0.fromCity }
let destinationCities = itinerary1.map { $0.toCity }

var travelLog: [String: Int] = [:]

originCities.forEach {
    if let x = travelLog[$0] {
        travelLog[$0] = x - 1
    } else {
        travelLog[$0] = -1
    }
}

destinationCities.forEach {
    if let y = travelLog[$0] {
        travelLog[$0] = y + 1
    } else {
        travelLog[$0] = 1
    }
}

let cities = travelLog.filter { $0.1 != 0 }
print(cities)

// Test cases

let itinerary1 = [
    Flight(fromCity: "DEN", toCity: "SEA"),
    Flight(fromCity: "SJC", toCity: "SLC"),
    Flight(fromCity: "SLC", toCity: "DEN"),
    Flight(fromCity: "LAX", toCity: "LAS"),
    Flight(fromCity: "SEA", toCity: "JFK"),
    Flight(fromCity: "LAS", toCity: "SJC"),
    Flight(fromCity: "SFO", toCity: "LAX"),
    Flight(fromCity: "ATL", toCity: "SFO")
] // ATL, SFO, LAX, LAS, SJC, SLC, DEN, SEA, JFK

let itinerary2 = [
    Flight(fromCity: "SFO", toCity: "LAX"),
    Flight(fromCity: "DEN", toCity: "SEA"),
    Flight(fromCity: "MIA", toCity: "SFO"),
    Flight(fromCity: "SFO", toCity: "MIA"),
    Flight(fromCity: "LAX", toCity: "LAS"),
    Flight(fromCity: "LAS", toCity: "SJC"),
    Flight(fromCity: "IAH", toCity: "DFW"),
    Flight(fromCity: "DFW", toCity: "MIA"),
    Flight(fromCity: "SJC", toCity: "SLC"),
    Flight(fromCity: "MIA", toCity: "IAH"),
    Flight(fromCity: "SLC", toCity: "DEN"),
    Flight(fromCity: "ATL", toCity: "SFO"),
    Flight(fromCity: "SEA", toCity: "JFK")
] // ATL, SFO, MIA, IAH, DFW, MIA, SFO, LAX, LAS, SJC, SLC, DEN, SEA, JFK

let itinerary3 = [
    Flight(fromCity: "LAX", toCity: "SFO"),
    Flight(fromCity: "SFO", toCity: "IAH"),
    Flight(fromCity: "IAH", toCity: "SEA"),
    Flight(fromCity: "SEA", toCity: "SFO"),
    Flight(fromCity: "SFO", toCity: "PHX"),
    Flight(fromCity: "PHX", toCity: "JFK"),
    Flight(fromCity: "JFK", toCity: "SFO")
] // LAX, SFO, IAH, SEA, SFO, PHX, JFK, SFO

let itinerary4 = [
    Flight(fromCity: "LAX", toCity: "SFO"),
    Flight(fromCity: "SFO", toCity: "IAH"),
    Flight(fromCity: "IAH", toCity: "SEA"),
    Flight(fromCity: "SEA", toCity: "SFO"),
    Flight(fromCity: "SFO", toCity: "PHX"),
    Flight(fromCity: "PHX", toCity: "JFK"),
    Flight(fromCity: "JFK", toCity: "SFO"),
    Flight(fromCity: "SFO", toCity: "ORL"),
    Flight(fromCity: "ORL", toCity: "SAC"),
    Flight(fromCity: "SAC", toCity: "SEA"),
    Flight(fromCity: "SEA", toCity: "SJC"),
    Flight(fromCity: "SJC", toCity: "SAC"),
    Flight(fromCity: "SAC", toCity: "MIA"),
    Flight(fromCity: "MIA", toCity: "SLC"),
    Flight(fromCity: "SLC", toCity: "SJC"),
    Flight(fromCity: "SJC", toCity: "DEN"),
    Flight(fromCity: "DEN", toCity: "LAX")
] // LAX, SFO, IAH, SEA, SFO, PHX, JFK, SFO, ORL, SAC, SEA, SJC, SAC, MIA, SLC, SJC, DEN, LAX