Implement Stack using Queues 用队列实现栈-优快云博客

本文链接：https://blog.youkuaiyun.com/chenyx90/article/details/50412152

本文详细介绍了如何利用两个队列来实现栈的基本操作：push、pop、top和empty。通过复制队列来确保栈的先进后出特性得以实现。

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Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.

Notes:

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

class MyStack {
    Queue<Integer> q1=new LinkedList<Integer>();//LinkedList类实现了Queue接口，ArrayList不行
    Queue<Integer> q2=new LinkedList<Integer>();
    Queue<Integer> q=new LinkedList<Integer>();
    // Push element x onto stack.
    public void push(int x) {
            q1.offer(x);
    }

    // Removes the element on top of the stack.
    public void pop() {
        while(q1.size()>1) q2.offer(q1.poll());

        q1.remove();
        q=q2;//如果不拷贝的话，在实现连续删除元素的时候就会出错
        q2=q1;
        q1=q;
    }

    // Get the top element.
    public int top() {
        while(q1.size()>1) q2.offer(q1.poll());

        int x=q1.poll();
        q2.offer(x);

        q=q2;//如果不拷贝的话，在实现连续top元素的时候就会出错
        q2=q1;
        q1=q;

        return x;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q1.isEmpty();
    }
}

class MyStack {
    Queue<Integer> q1=new LinkedList<Integer>();//LinkedList类实现了Queue接口，ArrayList不行
    Queue<Integer> q2=new LinkedList<Integer>();
    // Push element x onto stack.
    public void push(int x) {
        while(q1.size()>0)   q2.offer(q1.poll());//在刚开始push的时候就实现最开始插入队列的元素排在队尾，这样之后的操作就直接当做栈的特性一步操作了
        q1.offer(x);

        while(q2.size()>0) q1.offer(q2.poll());
        }

    // Removes the element on top of the stack.
    public void pop() {
        q1.remove();
    }

    // Get the top element.
    public int top() {
        return q1.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q1.isEmpty();
    }
}