update Stack implementation to Swift 4 with Xcode 9b4
update Stack implementation to Swift 4 with Xcode 9b4

raywenderlich/swift-algorithm-club,jawwad/swift-algorithm-club,hollance/swift-algorithm-club,raywenderlich/swift-algorithm-club,jawwad/swift-algorithm-club,jawwad/swift-algorithm-club,raywenderlich/swift-algorithm-club,tkester/swift-algorithm-club,tkester/swift-algorithm-club