力扣-用队列实现栈

题目描述：

仅使用两个队列实现一个后入先出（LIFO）的栈，并支持普通队列的全部四种操作（push、top、pop 和 empty）。

其中：
push：将元素 x 压入栈顶
pop：移除并返回栈顶元素
top：返回栈顶元素
empty：如果栈是空的，返回 true ；否则，返回 false

参考代码：

class MyStack {private Queue<Integer> queue1;private Queue<Integer> queue2;/** Initialize your data structure here. */public MyStack() {queue1 = new LinkedList<>();queue2 = new LinkedList<>();}/** Push element x onto stack. */public void push(int x) {if (!queue1.isEmpty()){queue1.offer(x);}else if (!queue2.isEmpty()){queue2.offer(x);}else {queue1.offer(x);}}/** Removes the element on top of the stack and returns that element. */public int pop() {if (empty()){return -1;}int qSize1 = queue1.size();int qSize2 = queue2.size();int data = 0;if (!queue1.isEmpty()){for (int i = 0; i < qSize1-1; i++) {queue2.offer(queue1.poll());}data = queue1.poll();}else {for (int i = 0; i < qSize2-1; i++) {queue1.offer(queue2.poll());}data = queue2.poll();}return data;}/** Get the top element. 拿到栈顶元素*/public int top() {if (empty()){return -1;}int qSize1 = queue1.size();int qSize2 = queue2.size();int data = 0;if (!queue1.isEmpty()){for (int i = 0; i < qSize1; i++) {data = queue1.poll();queue2.offer(data);}}else {for (int i = 0; i < qSize2; i++) {data = queue2.poll();queue1.offer(data);}}return data;}/** Returns whether the stack is empty. */public boolean empty() {if(queue1.isEmpty() && queue2.isEmpty()){return true;}return false;}
}

运行截图：