Swift-----泛型Generic

struct Stack<T> {

    var items = Array<T>()
    
    func isEmpty() -> Bool {
        return items.count == 0
    }
    
    mutating func push(value: T) {
        items.append(value)
    }
    
    mutating func pop() -> T? {
        
        return items.removeLast()
    }
}

// 对Stack进行扩展
extension Stack {

    var top: T? {
        return items.last
    }
}

var stack = Stack<Int>()
stack.push(value: 1)
stack.push(value: 2)
stack.pop()

var s = Stack<String>()
s.push(value: "Hello")
s.pop()
s.top