栈实现队列，队列实现栈

队列实现栈思路：移动到只有一个数，然后交换一下

public static class TwoStacksQueue {
		public Stack<Integer> stackPush;
		public Stack<Integer> stackPop;

		public TwoStacksQueue() {
			stackPush = new Stack<Integer>();
			stackPop = new Stack<Integer>();
		}

		// push栈向pop栈倒入数据
		private void pushToPop() {
			if (stackPop.empty()) {
				while (!stackPush.empty()) {
					stackPop.push(stackPush.pop());
				}
			}
		}

		public void add(int pushInt) {
			stackPush.push(pushInt);
			pushToPop();
		}

		public int poll() {
			if (stackPop.empty() && stackPush.empty()) {
				throw new RuntimeException("Queue is empty!");
			}
			// pushToPop();
			return stackPop.pop();
		}

		public int peek() {
			if (stackPop.empty() && stackPush.empty()) {
				throw new RuntimeException("Queue is empty!");
			}
			// pushToPop();
			return stackPop.peek();
		}
	}
	public static class TwoQueueStack<T> {
		public Queue<T> queue;
		public Queue<T> help;

		public TwoQueueStack() {
			queue = new LinkedList<>();
			help = new LinkedList<>();
		}

		public void push(T value) {
			queue.offer(value);
		}

		public T poll() {
			while (queue.size() > 1) {
				help.offer(queue.poll());
			}
			T ans = queue.poll();
			Queue<T> tmp = queue;
			queue = help;
			help = tmp;
			return ans;
		}

		public T peek() {
			while (queue.size() > 1) {
				help.offer(queue.poll());
			}
			T ans = queue.poll();
			help.offer(ans);
			Queue<T> tmp = queue;
			queue = help;
			help = tmp;
			return ans;
		}

		public boolean isEmpty() {
			return queue.isEmpty();
		}

	}