I created an Xcode project with my changes. It makes it easier to

[rzaccone]

I created an Xcode project with my changes. It makes it easier to test and the playground was crashing!

[kelvinlauKL]

Haven't gone through everything yet, but here's some preliminary feedback that should warrant some discussion / modifications.

[kelvinlauKL]

A for loop can do exactly what we're trying to do in a cleaner manner in this scenario (I know the site tutorial uses the while construct, but a fellow contributor alerted me of a better way).

for character in word.lowercased().characters {
  if let childNode = currentNode.children[character] {
    currentNode = childNode
  } else {
    currentNode.add(value: character)
    currentNode = currentNode.children[character]!
  }
}

This removes the necessity of the index and charactersInWord properties. The for construct also helps us bind each element in the word.lowercased().characters, alleviating the necessity of picking it out ourselves.

[rzaccone]

I agree. This is a good change.

[kelvinlauKL]

Unnecessary reference to self.

[kelvinlauKL]

Replacing this with a for loop would simplify things here too:

func contains(word: String) -> Bool {
  guard !word.isEmpty else {
    return false
  }
    
  var currentNode = root
  for character in word.lowercased().characters {
    guard let childNode = currentNode.children[character] else { return false }
    currentNode = childNode
  }
    
  return currentNode.isTerminating
}

[rzaccone]

Again, I agree.

[rzaccone]

Here's a slight simplification to the for loop in wordsInSubtrie.

        for childNode in rootNode.children.values {
            let childWords = wordsInSubtrie(rootNode: childNode, partialWord: previousLetters)
            subtrieWords += childWords
        }

[rzaccone]

Along the lines of your other good suggestions, here's an improvement to findTerminalNodeOf. It's the same adjustment to the for loop.

    private func findTerminalNodeOf(word: String) -> Node? {
        var currentNode = root
        for character in word.lowercased().characters {
            guard let childNode = currentNode.children[character] else {
                return nil
            }
            currentNode = childNode
        }
        return currentNode.isTerminating ? currentNode : nil
    }

[kelvinlauKL]

Yep, things are looking pretty Swifty :)

Could you update the PR with the changes?

[rzaccone]

I pushed all my changes and all tests pass.

[kelvinlauKL]

Looks great. I took a look over everything, and all tests passed.

I'll merge this in now. Cheers!