题目

Implement the following operations of a queue using stacks.

• push(x) — Push element x to the back of queue.
• pop() — Removes the element from in front of queue.
• peek() — Get the front element.
• empty() — Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack — which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

解题思路

题目要求用栈实现队列，但是go语言没有实现栈，所以还要先实现栈，这里使用slice实现栈
栈是先进后出，队列是先进先出，所以使用两个栈，两次先进后出，就变成了先进先出
例如：
一列数1，2，3，4，5，先放入一个栈中再出栈的顺序为5，4，3，2，1，出栈后放入另一个队列再出栈的顺序就变为1，2，3，4，5

代码

type MyStack struct {
    val []int
}

func ConstructorStack() MyStack  {
    return MyStack{[]int{}}
}

func (this *MyStack) Push(x int) {
    this.val = append([]int{x}, this.val...)
}

func (this *MyStack) Pop() int {
    tmp := this.val[0]
    this.val = this.val[1:]
    return tmp
}

func (this *MyStack) Peek() int {
    return this.val[0]
}
func (this *MyStack) Size() int {
    return len(this.val)
}

func (this *MyStack) Empty() bool {
    if 0 == len(this.val) {
        return true
    }

    return false
}

type MyQueue struct {
    stack1 MyStack
    stack2 MyStack
}


/** Initialize your data structure here. */
func Constructor() MyQueue {
    var queue MyQueue
    queue.stack1 = ConstructorStack()
    queue.stack2 = ConstructorStack()

    return queue
}


/** Push element x to the back of queue. */
func (this *MyQueue) Push(x int)  {
    this.stack1.Push(x)
}


/** Removes the element from in front of queue and returns that element. */
func (this *MyQueue) Pop() int {
    if !this.stack2.Empty() {
        return this.stack2.Pop()
    } else {
        for ;!this.stack1.Empty(); {
            tmp := this.stack1.Pop()
            this.stack2.Push(tmp)
        }
        return this.stack2.Pop()
    }
}


/** Get the front element. */
func (this *MyQueue) Peek() int {
    if !this.stack2.Empty() {
        return this.stack2.Peek()
    } else {
        for ;!this.stack1.Empty(); {
            tmp := this.stack1.Pop()
            this.stack2.Push(tmp)
        }
        return this.stack2.Peek()
    }
}


/** Returns whether the queue is empty. */
func (this *MyQueue) Empty() bool {
    return this.stack1.Empty() && this.stack2.Empty()
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(x);
 * param_2 := obj.Pop();
 * param_3 := obj.Peek();
 * param_4 := obj.Empty();
 */