# 225. Implement Stack using Queues

#### Easy

***

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (`push`, `top`, `pop`, and `empty`).

Implement the `MyStack` class:

* `void push(int x)` Pushes element x to the top of the stack.
* `int pop()` Removes the element on the top of the stack and returns it.
* `int top()` Returns the element on the top of the stack.
* `boolean empty()` Returns `true` if the stack is empty, `false` otherwise.

**Notes:**

* You must use **only** standard operations of a queue, which means that only `push to back`, `peek/pop from front`, `size` and `is empty` operations are valid.
* Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

 

**Example 1:**

```
Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False
```

 

**Constraints:**

* `1 <= x <= 9`
* At most `100` calls will be made to `push`, `pop`, `top`, and `empty`.
* All the calls to `pop` and `top` are valid.

&#x20;

**Follow-up:** Can you implement the stack using only one queue?

#### Solution 1 :&#x20;

```python
class MyStack:

    def __init__(self):
        self.q = []
        self.temp = []
        

    def push(self, x: int) -> None:
        self.q.append(x)
        
    def pop(self) -> int:
        for index in range(len(self.q)-1):
            self.temp.append(self.q[index])
        value = self.q[-1]
        self.q = self.temp
        self.temp = []
        return value
        

    def top(self) -> int:
        return self.q[-1]
        

    def empty(self) -> bool:
        return len(self.q) == 0
        


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

#### Solution 2 :&#x20;

```python
class MyStack:

    def __init__(self):
        self.q = deque()
        

    def push(self, x: int) -> None:
        self.q.append(x)
        for _ in range(len(self.q)-1):
            self.q.append(self.q.popleft())
        
    def pop(self) -> int:
        return self.q.popleft()
        

    def top(self) -> int:
        return self.q[0]
        

    def empty(self) -> bool:
        return len(self.q) == 0
        


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