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).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.

思路分析：这题和相似，思路也相似。用两个队列来模拟一个栈。详细做法是。入栈仅仅须要放入Q1队列尾部。出栈时。把Q1除了最后一个元素之外的全部元素出队列。而且压入Q2队列尾部。然后从Q1中取出最后那个元素。注意要保证Q2为空，它是一个辅助队列。所以我们交换Q1和Q2。

top和pop方法相似，除了取出Q1中最后那个元素后，再返回它前还要压入到Q2中去。保证这个元素不被丢失，由于我们仅仅想看栈顶元素，并不想真正将它出栈。

AC Code

class MyStack {       //Queue   LinkedList
     
       q1 = new LinkedList
      
       ();   LinkedList
       
         q2 = new LinkedList
        
         ();       // Push element x onto stack.    public void push(int x) {        q1.add(x);    }    // Removes the element on top of the stack.    public void pop() {        //peek(); poll();        while(q1.size() > 1){		    q2.add(q1.poll());    	}	    q1.poll();	    //switch 	    LinkedList
         
           tem = q1;	    q1 = q2;	    q2 = tem;    }    // Get the top element.    public int top() {	    //peek(); poll();        while(q1.size() > 1){		q2.add(q1.poll());    	}    	int res = q1.peek();    	q2.add(q1.poll());    	//switch     	LinkedList
          
            tem = q1; q1 = q2; q2 = tem; return res; } // Return whether the stack is empty. public boolean empty() { return q1.isEmpty(); }}