Read How to Prepare for a Technical Interview at Facebook I was caught up with solution:

func isStringUnique(_ s: String) -> Bool {
    return s.count == Set(s).count

That’s why I have jumped to source of Swift standard library to check complexity of Set.init(_: Array).

Side note, why they use both Dictionary<Key, Value> and [Key : Value] in same source file: mutating func remove(at index: Dictionary<Key, Value>.Index) -> [Key : Value].Element and mutating func removeValue(forKey key: [Key : Value].Key) -> [Key : Value].Value? It sounds like a good point for experimentation with compiling Swift.

Interesting comment to lazy property of Dictionary:

/// A view onto this collection that provides lazy implementations of
/// normally eager operations, such as `map` and `filter`.
/// Use the `lazy` property when chaining operations to prevent
/// intermediate operations from allocating storage, or when you only
/// need a part of the final collection to avoid unnecessary computation.
public var lazy: LazyCollection<Dictionary<Key, Value>> { get }

Swift standard library worth looking into all header files!!!

Again, caught with comment

/// Creates a new set from a finite sequence of items.

Are there infinite sequences?

And I haven’t found answer about complexity of Set.init(_: Array).

Returning to isStringUnique, there’s follow up Code Challenge: Using ASCII & Unicode with Swift. I didn’t get why his THE UNICODE APPROACH implementation is correct. In particular, guard to count less then 128. Or I am missing something or this implementation is incorrect. I have checked everything in playground and have an example of a string which produces wrong result.