Airbnb - findTag

func findTag(str: String, tags: [String: String]) -> String{
    var res = ""
    let trie = Trie()
    for tag in tags.keys{
        trie.addWord(tag)
    }
    var i = 0
    var arr = Array(str)
    while i < arr.count {
        if trie.hasPrefix(String(arr[i]).lowercased()){
            var j = i
            while j < arr.count{
                if !trie.hasPrefix(String(arr[i ... j]).lowercased()){
                    res.append(arr[i])
                    i += 1
                    break
                }
                if trie.hasWord(String(arr[i ... j]).lowercased()){
                    res.append("[\(String(arr[i ... j])) : \(tags[String(arr[i ... j]).lowercased()]!)]")
                    //arr.replaceSubrange(Range(NSMakeRange(i, j - i))!, with: tags[String(arr[i ... j])]!)
                    i = j + 1
                    break
                }
                j += 1
            }
        }else{
            res.append(arr[i])
            i += 1
        }
    }
    return res
}

class Trie {
    var root = Node()
    func addWord( _ str: String){
        var runner = root
        for char in str{
            if let childN = runner.child[char]{
                runner = childN
            }else{
                runner.child[char] = Node()
                runner = runner.child[char]!
            }
        }
        runner.hasWord = true
    }
    func hasPrefix(_ pre: String) -> Bool{
        var runner = root
        for char in pre{
            if let childN = runner.child[char]{
                runner = childN
            }else{
                return false
            }
        }
        return true
    }
    func hasWord(_ word : String) -> Bool{
        var runner = root
        for char in word{
            if let childN = runner.child[char]{
                runner = childN
            }else{
                return false
            }
        }
        return runner.hasWord
    }
}

class Node{
    var child = [Character : Node]()
    var hasWord = false
}



let tag =  ["airbnb": "business", "san francisco": "city"]
let str = "I visited San Francisco for work and stayed at Airbnb. I really loved the city and the home where I stayed."
let res = findTag(str: str, tags: tag)
print(res)