Exercise 2 solution

    def unique_in_order(items):
        return list(dict.fromkeys(items))

    def unique_in_order_set(items):
        seen = set()
        out = []
        for x in items:
            if x not in seen:
                seen.add(x)
                out.append(x)
        return out

    data = [3, 1, 3, 2, 1, 4]
    print(unique_in_order(data), unique_in_order_set(data))

Output:
    [3, 1, 2, 4] [3, 1, 2, 4]

list(set(items)) removes duplicates but a set makes no promise about order,
so the first-seen order is not guaranteed. A dict does guarantee insertion
order, and dict.fromkeys keeps only the first occurrence of each key.