Stack.swift

class Stack {

  typealias Element = Int

  private var storage: [Element] = []
  private var minimumList: [Element] = []

  func push(item: Element) {
    let minOfCurrentAndLast = min(item, minimum() ?? item)
    minimumList.append(minOfCurrentAndLast)

    storage.append(item)
  }

  func pop() -> Element? {
    guard !storage.isEmpty else {
      return nil
    }

    minimumList.removeLast()

    return storage.removeLast()
  }

  func minimum() -> Element? {
    return minimumList.last
  }

}