###### tags: `stack`, `Design`, `Quene`

# LeetCode 232.Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (```push```, ```peek```, ```pop```, and ```empty```).

Implement the MyQueue class:

- ```void push(int x)``` Pushes element x to the back of the queue.
- ```int pop()``` Removes the element from the front of the queue and returns it.
- ```int peek()``` Returns the element at the front of the queue.
- ```boolean empty()``` Returns ```true``` if the queue is empty, ```false``` otherwise.

**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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

>Example 1:

Input:
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output:
[null, null, null, 1, 1, false]

Explanation:
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

### Constraints:

- $1 <= x <= 9$
- At most 100 calls will be made to push, pop, peek, and empty.
- All the calls to pop and peek are valid..
---
### Idea:
>用 list 模擬 stack 操作，刪除的時候利用一個 stack pop 到另一個 stack push 這樣就可以刪除先進來的值，接著再做一次同樣的操作就可以把 stack 還原回去原本的順序。
### Solution:

Python:

class MyQueue:

    def __init__(self):
        self.stack = []
        self.re = []
    def push(self, x: int) -> None:
        self.stack.append(x)
    def pop(self) -> int:
        while (self.stack!=[]):
            self.re.append(self.stack.pop())
        delt = self.re.pop()
        while (self.re!=[]):
            self.stack.append(self.re.pop())
        return delt
    def peek(self) -> int:
        return self.stack[0]
    def empty(self) -> bool:
        return self.stack == []

# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

C++: