232. 用栈实现队列

使用两个栈，注意pop和peek：

class MyQueue {private Stack<Integer> stackIn;private Stack<Integer> stackOut;public MyQueue() {stackIn = new Stack<>();stackOut = new Stack<>();}public void push(int x) {stackIn.push(x);}/** Removes the element from in front of queue and returns that element. */public int pop() {dumpstackIn();return stackOut.pop();}/** Get the front element. */public int peek() {dumpstackIn();return stackOut.peek();}/** Returns whether the queue is empty. */public boolean empty() {return stackIn.isEmpty() && stackOut.isEmpty();}// 如果stackOut为空，那么将stackIn中的元素全部放到stackOut中private void dumpstackIn(){if (!stackOut.isEmpty()) return;while (!stackIn.isEmpty()){stackOut.push(stackIn.pop());}}
}

(这里记得在构造器中java要把两个栈实例化，cpp不用）

225. 用队列实现栈

class MyStack {Queue<Integer> queue;public MyStack() {queue =  new LinkedList<>();}public void push(int x) {queue.add(x);}public int pop() {int len = queue.size();while(len > 1){queue.add(queue.poll());len--;}return queue.poll();}public int top() {int topElement = pop();queue.add(topElement);return topElement;}public boolean empty() {return queue.isEmpty();}