225. Implement Stack using Queues

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.

• 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).

可以用兩個Queue來輪流儲存資料，當執行pop，top等指令時，因為要返回Queue中最後一個element，所以把之前的所有elements壓到另外一個空queue中，剩下唯一一個element的時候，實現poll返回。依次交替

注意事項：pop，top等取出行為，要先檢查兩個queue是不是都為空

import java.util.LinkedList;
import java.util.Queue;

public class MyStack {
	Queue<Integer> q1 = new LinkedList<Integer>();
	Queue<Integer> q2 = new LinkedList<Integer>();

	/** Initialize your data structure here. */
	public MyStack() {

	}

	/** Push element x onto stack. */
	public void push(int x) {
		if (q2.isEmpty()) {
			q1.offer(x);
		} else
			q2.offer(x);

	}

	/** Removes the element on top of the stack and returns that element. */
	public int pop() {
		while (!q2.isEmpty()) {
			if (q2.size() == 1)
				return q2.poll();
			q1.offer(q2.poll());
		}
		while (!q1.isEmpty()) {
			if (q1.size() == 1)
				return q1.poll();
			q2.offer(q1.poll());
		}
		return -1;
	}

	/** Get the top element. */
	public int top() {
		while (!q2.isEmpty()) {
			if (q2.size() == 1) {
				int x = q2.peek();
				q1.offer(q2.poll());
				return x;
			}

			q1.offer(q2.poll());
		}
		while (!q1.isEmpty()) {
			if (q1.size() == 1) {
				int x = q1.peek();
				q2.offer(q1.poll());
				return x;
			}
			q2.offer(q1.poll());
		}
		return -1;
	}

	/** Returns whether the stack is empty. */
	public boolean empty() {
		return q1.isEmpty() && q2.isEmpty();
	}
}