Swift-两个栈实现队列

题目:两个栈实现队列，栈是先入后出，队列是先入先出，两个栈可以利用这个特点实现队列.

核心代码:

`class MyQueue {

var stackNew:[Int] = []
var stackOld:[Int] = []

func push(value:Int) {
    stackNew.append(value)
}

func peek() -> Int? {
    shiftStacks()
    let value:Int? = stackOld.last
    if value != nil {
        stackOld.removeLast()
    }
    return value
}

private func shiftStacks() {
    if stackOld.count == 0 {
        while stackNew.count > 0 {
            let value:Int = stackNew.last!
            stackNew.removeLast()
            stackOld.append(value)
        }
    }
}

}`

测试代码:

`var myQueue:MyQueue = MyQueue()
 for i in 0...3 {
 myQueue.push(value: i)
 }

var topValue:Int? = myQueue.peek()
 if topValue != nil {
 print("FlyElephant---顶部数据:(topValue!)")
 }
for i in 10...15 {
 myQueue.push(value: i)
 }
for i in 0...4 {
 var topValue:Int? = myQueue.peek()
 if topValue != nil {
 print("FlyElephant---顶部数据:(topValue!)")
 }
 }`

FlyElephant.png