# Implement Queue using Stacks

### 描述​

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).

### 分析​

• push(x)，先将s中的元素全部弹出来，存入tmp，把x push 到tmp，然后把tmp中的元素全部弹出来，存入s
• pop()，直接将s的栈顶元素弹出来即可

### 代码​

// Implement Queue using Stacksclass MyQueue {    // Push element x to the back of queue.    // Time Complexity: O(n)    public void push(int x) {        while (!s.isEmpty()) {            final int e = s.pop();            tmp.push(e);        }        tmp.push(x);        while(!tmp.isEmpty()) {            final int e = tmp.pop();            s.push(e);        }    }    // Removes the element from in front of queue.    // Time Complexity: O(1)    public void pop() {        s.pop();    }    // Get the front element.    public int peek() {        return s.peek();    }    // Return whether the queue is empty.    public boolean empty() {        return s.isEmpty();    }    private final Stack<Integer> s = new Stack<>();    private final Stack<Integer> tmp = new Stack<>();}