WordLadder.swift

import Foundation

let url = try URL(string: "http://norvig.com/ngrams/TWL06.txt").unwrap()

let data = try Data(contentsOf: url)

let string = try String(data: data, encoding: .utf8).unwrap()

let array = string.components(separatedBy: .newlines)

let lexicon = Set(array.fourLetterStrings())

let allLetters = Array("ABCDEFGHIJKLMNOPQRSTUVWXYZ")

func findPath(between sourceWord: String, and targetWord: String) -> [String]? {

    if sourceWord == targetWord { return [] }

    var seen: Set = [sourceWord]

    var queue: [(item: String, path: [String])] = [(sourceWord, [])]

    while true {
        if queue.count == 0 { return nil }
        let (item, path) = queue.removeFirst()
        if item == targetWord { return path + [item] }
        let sourceAsArray = Array(item)
        for characterIndex in sourceAsArray.indices {
            for letter in allLetters {
                var copy = sourceAsArray
                copy[characterIndex] = letter
                let oneLetterAway = String(copy)

                if lexicon.contains(oneLetterAway) && !seen.contains(oneLetterAway) {
                    queue.append((oneLetterAway, path + [item]))
                    seen.insert(oneLetterAway)
                }
            }
        }
    }
}

let path = try findPath(between: "TOON", and: "PLEA").unwrap()

print(path)