队列实现一个栈

当插入一个元素时，要实现后进元素先出，需要把该元素之前的元素先出队列，重新进入队列的尾部。此时就实现了后进先出

队列插入

class MyStack {
    Queue<Integer> queue;

    /** Initialize your data structure here. */
    public MyStack() {
        queue=new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        int size=queue.size();
        while(size-->1){
            //插入元素前面的元素先出来，在进入队列
            queue.add(queue.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.remove();
    }
    
    /** 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();
 */

一个元素默认尾部插入，应该让被插入的元素进入队列的首部，