Trie implementation for a pointless Boggle solver

boggle.swift

struct LetterNode {
    var isWord = false
    var children: [Character: LetterNode] = [:]

    mutating func insert(var word: String) {
        if word == "" {
            isWord = true
            return
        }

        let c = word[word.startIndex]
        if children[c] == nil {
            children[c] = LetterNode()
        }

        word = String(word.characters.dropFirst())
        children[c]!.insert(word)
    }
}

let wordsUrl = NSURL(fileURLWithPath: "/usr/share/dict/words")
let words = try String(contentsOfURL: wordsUrl, encoding: NSUTF8StringEncoding)

var wordTree = LetterNode()
for word in words.componentsSeparatedByString("\n") {
    wordTree.insert(word.lowercaseString)
}