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feat: add python and java solutions to lcci question
添加《程序员面试金典》题解：面试题 03.04. 化栈为队
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‎lcci/03.04.Implement Queue using Stacks/README.md

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## 解法
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<!-- 这里可写通用的实现逻辑 -->
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- 每次压入元素时，放入第 1 个栈中；
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- 第 2 个栈不为空时，不能倒入元素；
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- 第 2 个栈为空时，必须将第 1 个栈的所有元素按顺序倒入第 2 个栈中。
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### Python3
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```python
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class MyQueue:
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def __init__(self):
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"""
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Initialize your data structure here.
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"""
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self._s1, self._s2 = [], []
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def push(self, x: int) -> None:
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"""
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Push element x to the back of queue.
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"""
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self._s1.append(x)
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def pop(self) -> int:
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"""
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Removes the element from in front of queue and returns that element.
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"""
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if len(self._s2) == 0:
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while self._s1:
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self._s2.append(self._s1.pop())
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return self._s2.pop()
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def peek(self) -> int:
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"""
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Get the front element.
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"""
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if len(self._s2) == 0:
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while self._s1:
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self._s2.append(self._s1.pop())
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return self._s2[-1]
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def empty(self) -> bool:
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"""
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Returns whether the queue is empty.
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"""
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return len(self._s1) + len(self._s2) == 0
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# Your MyQueue object will be instantiated and called as such:
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# obj = MyQueue()
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# obj.push(x)
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# param_2 = obj.pop()
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# param_3 = obj.peek()
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# param_4 = obj.empty()
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```
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### Java
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```java
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class MyQueue {
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private Stack<Integer> s1;
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private Stack<Integer> s2;
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/** Initialize your data structure here. */
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public MyQueue() {
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s1 = new Stack<>();
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s2 = new Stack<>();
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}
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/** Push element x to the back of queue. */
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public void push(int x) {
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s1.push(x);
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}
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/** Removes the element from in front of queue and returns that element. */
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public int pop() {
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if (s2.empty()) {
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while (!s1.empty()) {
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s2.push(s1.pop());
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}
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}
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return s2.pop();
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}
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/** Get the front element. */
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public int peek() {
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if (s2.empty()) {
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while (!s1.empty()) {
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s2.push(s1.pop());
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}
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}
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return s2.peek();
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}
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/** Returns whether the queue is empty. */
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public boolean empty() {
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return s1.empty() && s2.empty();
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}
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}
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/**
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* Your MyQueue object will be instantiated and called as such:
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* MyQueue obj = new MyQueue();
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* obj.push(x);
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* int param_2 = obj.pop();
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* int param_3 = obj.peek();
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* boolean param_4 = obj.empty();
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*/
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```
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### ...

‎lcci/03.04.Implement Queue using Stacks/README_EN.md

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# [03.04. Implement Queue using Stacks](https://leetcode-cn.com/problems/implement-queue-using-stacks-lcci)
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## Description
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<p>Implement a MyQueue class which implements a queue using two stacks.</p>
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&nbsp;
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<p><strong>Example: </strong></p>
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<pre>
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MyQueue queue = new MyQueue();
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queue.push(1);
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queue.push(2);
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queue.peek(); // return 1
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queue.pop(); // return 1
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queue.empty(); // return false</pre>
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<p>&nbsp;</p>
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<p><b>Notes:</b></p>
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<ul>
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<li>You must use&nbsp;<i>only</i>&nbsp;standard operations of a stack -- which means only&nbsp;<code>push to top</code>,&nbsp;<code>peek/pop from top</code>,&nbsp;<code>size</code>, and&nbsp;<code>is empty</code>&nbsp;operations are valid.</li>
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<li>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.</li>
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<li>You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).</li>
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</ul>
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<p>&nbsp;</p>
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## Solutions
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### Python3
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```python
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```
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### Java
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```java
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```
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### ...
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```
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```
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# [03.04. Implement Queue using Stacks](https://leetcode-cn.com/problems/implement-queue-using-stacks-lcci)
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## Description
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<p>Implement a MyQueue class which implements a queue using two stacks.</p>
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&nbsp;
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<p><strong>Example: </strong></p>
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<pre>
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MyQueue queue = new MyQueue();
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queue.push(1);
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queue.push(2);
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queue.peek(); // return 1
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queue.pop(); // return 1
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queue.empty(); // return false</pre>
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<p>&nbsp;</p>
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<p><b>Notes:</b></p>
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<ul>
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<li>You must use&nbsp;<i>only</i>&nbsp;standard operations of a stack -- which means only&nbsp;<code>push to top</code>,&nbsp;<code>peek/pop from top</code>,&nbsp;<code>size</code>, and&nbsp;<code>is empty</code>&nbsp;operations are valid.</li>
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<li>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.</li>
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<li>You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).</li>
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</ul>
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<p>&nbsp;</p>
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## Solutions
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### Python3
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```python
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class MyQueue:
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def __init__(self):
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"""
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Initialize your data structure here.
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"""
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self._s1, self._s2 = [], []
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def push(self, x: int) -> None:
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"""
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Push element x to the back of queue.
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"""
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self._s1.append(x)
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def pop(self) -> int:
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"""
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Removes the element from in front of queue and returns that element.
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"""
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if len(self._s2) == 0:
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while self._s1:
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self._s2.append(self._s1.pop())
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return self._s2.pop()
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def peek(self) -> int:
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"""
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Get the front element.
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"""
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if len(self._s2) == 0:
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while self._s1:
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self._s2.append(self._s1.pop())
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return self._s2[-1]
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def empty(self) -> bool:
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"""
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Returns whether the queue is empty.
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"""
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return len(self._s1) + len(self._s2) == 0
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# Your MyQueue object will be instantiated and called as such:
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# obj = MyQueue()
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# obj.push(x)
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# param_2 = obj.pop()
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# param_3 = obj.peek()
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# param_4 = obj.empty()
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```
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### Java
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```java
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class MyQueue {
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private Stack<Integer> s1;
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private Stack<Integer> s2;
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/** Initialize your data structure here. */
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public MyQueue() {
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s1 = new Stack<>();
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s2 = new Stack<>();
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}
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/** Push element x to the back of queue. */
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public void push(int x) {
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s1.push(x);
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}
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/** Removes the element from in front of queue and returns that element. */
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public int pop() {
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if (s2.empty()) {
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while (!s1.empty()) {
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s2.push(s1.pop());
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}
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}
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return s2.pop();
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}
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/** Get the front element. */
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public int peek() {
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if (s2.empty()) {
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while (!s1.empty()) {
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s2.push(s1.pop());
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}
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}
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return s2.peek();
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}
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/** Returns whether the queue is empty. */
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public boolean empty() {
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return s1.empty() && s2.empty();
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}
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}
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/**
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* Your MyQueue object will be instantiated and called as such:
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* MyQueue obj = new MyQueue();
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* obj.push(x);
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* int param_2 = obj.pop();
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* int param_3 = obj.peek();
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* boolean param_4 = obj.empty();
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*/
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```
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### ...
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```
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```

‎lcci/03.04.Implement Queue using Stacks/Solution.java

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class MyQueue {
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private Stack<Integer> s1;
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private Stack<Integer> s2;
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/** Initialize your data structure here. */
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public MyQueue() {
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s1 = new Stack<>();
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s2 = new Stack<>();
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}
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/** Push element x to the back of queue. */
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public void push(int x) {
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s1.push(x);
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}
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/** Removes the element from in front of queue and returns that element. */
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public int pop() {
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if (s2.empty()) {
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while (!s1.empty()) {
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s2.push(s1.pop());
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}
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}
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return s2.pop();
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}
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/** Get the front element. */
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public int peek() {
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if (s2.empty()) {
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while (!s1.empty()) {
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s2.push(s1.pop());
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}
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}
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return s2.peek();
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}
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/** Returns whether the queue is empty. */
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public boolean empty() {
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return s1.empty() && s2.empty();
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}
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}
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/**
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* Your MyQueue object will be instantiated and called as such:
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* MyQueue obj = new MyQueue();
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* obj.push(x);
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* int param_2 = obj.pop();
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* int param_3 = obj.peek();
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* boolean param_4 = obj.empty();
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*/

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