Fibonacci top down passing the cache

FibTopDownPassCache

func fib (_ num: Int) -> Int {
    var first = [0, 1]
    return fib(num, &first)
}

func fib(_ num: Int, _ cache: inout [Int]) -> Int {
    if (cache.count > num) {
        return cache[num]
    }
    let fibNum = fib(num - 1, &cache) + fib(num - 2, &cache)
    cache.append(fibNum)
    return fibNum
}