Implement Queue using Stacks

Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.

Notes:

You must use only standard operations of a stack -- which means only

push
to top

,

peek/pop from top

,

size

,
and

is empty

operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue),
as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

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Have you met this question in a real interview

既然仍然可以用list模拟，先给出用list模拟的，再给出用stack模拟的。
代码如下：
list模拟队列

class MyQueue {
List<Integer> queue = new ArrayList<Integer>();
// Push element x to the back of queue.
public void push(int x) {
queue.add(x);
}

// Removes the element from in front of queue.
public void pop() {
if(!empty())
queue.remove(queue.get(0));
}

// Get the front element.
public int peek() {
return queue.get(0);
}

// Return whether the queue is empty.
public boolean empty() {
return queue.size()==0;//return queue.isEmpty(）也行
}
}

2个stack模拟队列如下：

class MyQueue {
Stack<Integer> s1 = new Stack<>();
Stack<Integer> s2 = new Stack<>();
// Push element x to the back of queue.
public void push(int x) {
s1.add(x);
}

// Removes the element from in front of queue.
public void pop() {
if(!s2.isEmpty()){
s2.pop();
}else{
while(!s1.isEmpty()){
s2.push(s1.pop());
}
s2.pop();//pop()有返回值，但是这里不接收即可，满足方法void的要求
}
}

// Get the front element.
public int peek() {
if(!s2.isEmpty()){
return s2.peek();
}else{
while(!s1.isEmpty()){
s2.push(s1.pop());
}
return s2.peek();
}
}

// Return whether the queue is empty.
public boolean empty() {
return s1.isEmpty()&&s2.isEmpty() ;
}
}