225. Implement Stack using Queues

Posted 2022-12-13 Carl_Hugo

https://leetcode.com/problems/min-stack/description/

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 only push to back, peek/pop from front, size, and is 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).

解题思路：
使用一个队列来模拟栈。

入队时，向队列中添加一个元素，并将队列中该元素前面的元素取出，添加到队列的后面。(将新添加的元素放在队列最前面)

出队时直接出队

public class MyStack 
Queue<Integer> queue;
    /** Initialize your data structure here. */
    public MyStack() 
        this.queue=new LinkedList<Integer>();
    

    /** Push element x onto stack. */
    public void push(int x) 
        queue.add(x);
        for(int i=0;i<queue.size()-1;i++)
            queue.add(queue.poll());
        
    

    /** Removes the element on top of the stack and returns that element. */
    public int pop() 
        return queue.poll();
    

    /** Get the top element. */
    public int top() 
        return queue.peek();
    

    /** Returns whether the stack is empty. */
    public boolean empty() 
        return queue.isEmpty()?true:false;
    


/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */