String characters count in Swift

func Reverse(word: String) -> String {
    let characters = Array(word.characters)
    //Characters count
    let charCount = word.characters.count

    var result = ""
    for (var i = charCount - 1; i >= 0; i--)
    {
        result += String(characters[i])
    }
    return result
}

var stringReverse = Reverse("string")
//stringReverse is "gnirts"