## Implement Stack using Queues

### 描述

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).

### 分析

• push(x)，先将x push 到tmp，然后把q中的元素全部弹出来，存入tmp，最后切换qtmp
• pop()，直接将q的队首元素弹出来即可

### 代码

// Implement Stack using Queues
class MyStack {
// Push element x onto stack.
// Time Complexity O(n)
public void push(int x) {
tmp.offer(x);
while (!q.isEmpty()) {
final int e = q.poll();
tmp.offer(e);
}
// swap q and tmp
Queue temp = tmp;
tmp = q;
q = temp;
}

// Removes the element on top of the stack.
// Time Complexity O(1)
public void pop() {
q.poll();
}

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

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

private Queue q = new LinkedList<>();
private Queue tmp = new LinkedList<>();
}