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)
    class MyStack {/*    本题是利用两个队列实现栈，有以下几种方法：    1、使用两个栈，当pop时，将其中之一队列的0~size()-2 add到备用队列，然后取第一队列的最后值，此种实现有两种方法：       (1)pop完备用队列可以再复制到第一队列       (2)增加一个cur标记位，指向当前的有效队列    2、如果使用一个队列实现，pop时需要计算此时队列中元素个数len，将len-1个元素add到队列的结尾，返回新的队首    3、使用一个队列，在push元素时，将队列中处在此元素之前的所有值都重新add到队尾，保证对头是最新加入的值，核心代码：       Q.push(x);        int size = Q.size() - 1;        for (int i = 0; i < size; i++) {            Q.push(Q.front());            Q.pop();        }*/    Map
             
              
               > temp=new HashMap
               
                
                 >();    {        LinkedList
                 
                   t=new LinkedList
                  
                   (); LinkedList
                   
                     s=new LinkedList
                    
                     (); temp.put(0,t); temp.put(1,s); }//注意放到块里，可以初始化 int cur=0; // Push element x onto stack. public void push(int x) { temp.get(cur).add(x); } // Removes the element on top of the stack. public void pop() { while(temp.get(cur).size()>1){ int t=temp.get(cur).remove(); temp.get(1-cur).add(t); } temp.get(cur).remove(); cur=1-cur; } // Get the top element. public int top() { while(temp.get(cur).size()>1){ int t=temp.get(cur).remove(); temp.get(1-cur).add(t); } int res=temp.get(cur).remove(); temp.get(1-cur).add(res); cur=1-cur; return res; } // Return whether the stack is empty. public boolean empty() { return temp.get(cur).isEmpty(); }}