Swift の問題　https://twitter.com/treastrain/status/1437058401083674625

main.swift

//
//  main.swift
//
//  Created by treastrain / Tanaka Ryoga on 2021/09/12.
//
//  These codes are licensed under CC0.
//  https://creativecommons.org/publicdomain/zero/1.0/deed
//

import Foundation

extension Array where Element: Collection, Element.Element: Equatable {
    func orderdSetWithFlag() -> [Element.Element?] {
        var elements = [Element.Element?]()
        self.forEach { collection in
            collection.forEach { element in
                if !elements.contains(element) {
                    if collection.first == element {
                        elements.append(nil)
                    }
                    elements.append(element)
                }
            }
        }
        elements.removeFirst()
        return elements
    }
}

let arrays1: [[Int]] = [
    [1, 2, 3, 4],
    [3, 4, 5, 6],
    [8, 9, 10, 11],
    [8, 9, 10, 11],
    [11, 12, 13, 14],
    [15, 16, 17, 18],
]
arrays1.orderdSetWithFlag().forEach {
    print($0 ?? "nil", terminator: "")
    print(", ", terminator: "")
} // 1, 2, 3, 4, 5, 6, nil, 8, 9, 10, 11, 12, 13, 14, nil, 15, 16, 17, 18,

let arrays2: [[String]] = [
    ["a", "b", "c", "d"],
    ["b", "c", "d", "e"],
    ["e", "f", "g", "h"],
    ["i", "j", "k", "l"],
    ["n", "o", "p", "q"],
]
arrays2.orderdSetWithFlag().forEach {
    print($0 ?? "nil", terminator: "")
    print(", ", terminator: "")
} // a, b, c, d, e, f, g, h, nil, i, j, k, l, nil, n, o, p, q,

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