225. Implement Stack using Queues

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本文介绍了一种使用两个队列来模拟栈的操作方法，包括push、pop、top和empty等基本操作。通过不断地将元素从一个队列转移到另一个队列中，实现了栈的后进先出特性。

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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.

用队列实现栈，队列先进先出，栈后进先出，使用两个队列来回把元素搬移，就实现了，假设队列为Q1，Q2，

如果栈依次进行以下操作：push(1),push(2),push(3)，相应的队列情况：

Q1	1	EMPTY	321
Q2	EMPTY	21	EMPTY

也就是每次找空的队列先把新元素入队，然后把另一个队列里所有元素出队并入队到当前队列，对应栈的push操作。

pop操作对应非空队列的出列操作。

class MyStack
{
	
	ArrayDeque<Integer> q1,q2;
	public MyStack()
	{
		q1=new ArrayDeque<>();
		q2=new ArrayDeque<>();
	}
	
	public void push(int x)
	{
		if(empty())
			q1.add(x);
		else
		{
			if(q1.isEmpty())
			{
				q1.add(x);
				while(!q2.isEmpty())
					q1.add(q2.poll());
			}
			else {
				q2.add(x);
				while(!q1.isEmpty())
					q2.add(q1.poll());
			}
		}
	}


	public void pop()
	{
		if(!empty())
		{
			if(!q1.isEmpty())
				q1.poll();
			else {
				q2.poll();
			}
		}
	}


	public int top()
	{
		if(!q1.isEmpty())
			return q1.peek();
			
		return q2.peek();
	}


	public boolean empty()
	{
		return q1.isEmpty()&&q2.isEmpty();
	}
}