[LEET CODE] 225. Implement Stack using Queues

# Problem

Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (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 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. 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.

 

# My Answer

class MyStack:

    def __init__(self):
        self.data=[]
        

    def push(self, x: int) -> None:
        self.data.append(x)
        

    def pop(self) -> int:
        a = self.data.pop()
        return a

    def top(self) -> int:
        """
        Get the top element.
        """
        return self.data[-1]
        
        

    def empty(self) -> bool:
        """
        Returns whether the stack is empty.
        """
        return len(self.data)==0

 

# Solution 1 - push() 할 때 큐를 이용해 재정렬

import collections

class MyStack:
    def __init__(self):
        self.q = collections.deque()

    def push(self, x):
        self.q.append(x)
        # 요소 삽입 후 맨 앞에 두는 상태로 재정렬
        for _ in range(len(self.q) - 1):
            self.q.append(self.q.popleft())

    def pop(self):
        return self.q.popleft()

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

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

큐의 FIFO에 해당하는 연산만 사용해서 구현

큐는 데크로 선언