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232. Implement Queue using Stacks

문제 링크

https://leetcode.com/problems/implement-queue-using-stacks/description/

문제 설명

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.

제한 사항

  • 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.

입출력 예 #1

  1. Input : [“MyQueue”, “push”, “push”, “peek”, “pop”, “empty”] [[], [1], [2], [], [], []]
  2. Output : [null, null, null, 1, 1, false]
  3. 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

문제 풀이

stack을 이용한 que 구현이기에 pop, peek 동작을 구현하기 위해 별도의 저장공간이 따로 필요하다.

class MyQueue:
    def __init__(self):
        self.input = []
        self.output = []

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

    def pop(self) -> int:
        self.peek()
        return self.output.pop()

    def peek(self) -> int:
        # output이 없으면 모두 재입력
        if not self.output:
            while self.input:
                self.output.append(self.input.pop())
        return self.output[-1]

    def empty(self) -> bool:
        return self.input == [] and self.output == []

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