LeetCode 225. 用队列实现栈（Java代码）

https://leetcode-cn.com/problems/implement-stack-using-queues/

题目描述

题解

• 使用一个队列实现即可
• 每次向队列中加入元素的时候，将队列中之前的元素全部出队列再入队列
• 这样最新入队列的元素，变成了第一个，第一个出队列。后入先出

class MyStack {

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

    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        int size = queue.size();
        queue.offer(x);
        // 将队列中原先存在的元素全部重新入一次队列
        for(int i=0; i<size; i++){
            queue.offer(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();
    }
}

/**
 * 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();
 */