### 题目

Implement the following operations of a stack using queues.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• empty() – Return whether the stack is empty.

Notes: You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid. Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue. You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

### 先全部倒出来，再插入

#### 代码

public class MyStack {

/** * Push element x onto stack.  O(n) */
public void push(int x) {
while (!q.isEmpty()) { buffer.offer(q.poll()); }
q.offer(x);
while (!buffer.isEmpty()) { q.offer(buffer.poll()); }
}

/** Removes the element on top of the stack and returns that element. */
public int pop() {
return q.poll();
}

/** Get the top element. */
public int top() {
return q.peek();
}

/** Returns whether the stack is empty. */
public boolean empty() {
return q.isEmpty();
}

}


### 可以不使用额外辅助容器

#### 代码

public class MyStack {

/** * Push element x onto stack.  O(n) */
public void push(int x) {
int size = q.size();
q.offer(x);
for (int i = 0; i < size; i++) {
q.offer(q.poll());
}
}

/** Removes the element on top of the stack and returns that element. */
public int pop() {
return q.poll();
}

/** Get the top element. */
public int top() {
return q.peek();
}

/** Returns whether the stack is empty. */
public boolean empty() {
return q.isEmpty();
}

}