Fibonacci top down

FibTopDownExternalCache

var cache : [Int] = [0]

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