Swift——数组

Array 实现栈

struct Stack {
      private var stack: [Element]
      var isEmpty: Bool { return stack.isEmpty }
        var peek: AnyObject? { return stack.last as AnyObject? }

      init() {
        stack = [Element]()
      }

      mutating func push(_ element: Element) {
        stack.append(element)
      }

        mutating func pop() -> Element? {
        return stack.popLast()
      }
    }

swift4.0 关键字 mutating
可以修饰值类型（struct，enum）内实现的方法，允许在方法内修改struct，enum 中定义的属性。