Iterative Stack Sum.swift

struct Stack<T> {
    private var storage = [T]()
    
    mutating func push(_ item: T) {
        storage.append(item)
    }
    
    mutating func pop() -> T? {
        return storage.removeLast()
    }
    
    func peek() -> T? {
        return storage.last
    }
    
    var count: Int {
        return storage.count
    }
}

final class SumOperationManager {
    
    private var sumOperations = Stack<[Any]>()
    
    func sum(_ collection: [Any]) -> Int {
        sumOperations.push(collection)
        
        var sum = 0
        
        repeat {
            guard let items = sumOperations.pop() else { return sum }
            
            for item in items {
                if let item = item as? Int {
                    sum += item
                } else if let item = item as? [Any] {
                    sumOperations.push(item)
                }
            }
        } while sumOperations.count > 0
        
        return sum
    }
}

let op = SumOperationManager()
op.sum([1, 1, 1, [1, 1, [1]]])