兩個棧實現一佇列：

要加入輔助棧，進行反轉操作

棧的順序為後進先出，而佇列的順序為先進先出。使用兩個棧實現佇列，一個元素需要經過兩個棧才能出佇列，在經過第一個棧時元素順序被反轉，經過第二個棧時再次被反轉，此時就是先進先出順序。

class MyQueue {
    Stack<Integer> in=new Stack<>();
    Stack<Integer> out=new Stack<>();

    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    //先入棧
    public void push(int x) {
        in.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    //要遵循佇列的先入先出，應該讓元素從入棧中先進入輔助棧，再出棧
    public int pop() {
        inToOut();
        return out.pop();
    }
    //執行出棧入棧操作
     public void inToOut() {
         if(out.isEmpty()){
           while(!in.isEmpty()){
               out.push(in.pop());
           }
         }
    }
    
    /** Get the front element. */
    public int peek() {
        inToOut();
      return  out.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return in.isEmpty()&&out.isEmpty();
    }
}

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