225. Implement Stack Using Queues

Edited by Charles

Easy · Python · Stack

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

Assume the input is [1, 2, 3] in order, and 3 now is the top of the stack. For pop(), we need to move 3 to the left and 1, 2 reinput to the queue from right.

Original study notes follow, including intermediate snippets and earlier attempts.

Note:

1. Assume the input is [1, 2, 3] in order, and 3 now is the top of the stack.
2. For pop(), we need to move 3 to the left and 1, 2 reinput to the queue from right.
3. For top(), we need to initialize a top_element in the beginning of the class to record the top element in the stack, so that we can retrieve top anytime.

from collections import deque

class MyStack:

    def __init__(self):
        self.queue = deque()
        self.top_element = 0

    def push(self, x: int) -> None:
        self.queue.append(x)
        self.top_element = x

    def pop(self) -> int:
        size = len(self.queue)

        while size > 1:
            self.queue.append(self.queue.popleft())
            size -= 1

        self.top_element = self.queue[-1]

        return self.queue.popleft()

    def top(self) -> int:
        return self.top_element

    def empty(self) -> bool:
        return not self.queue

# 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()

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 9, 2026
Feedback	Strongly Recommended for Multiple Attempts

Python solution

# from collections import deque
class MyStack:
    from collections import deque
    # q1 = [ ]
    # q2 = [ ]
    def __init__(self):
        self.q1 = deque()
        self.q2 = deque()

    def push(self, x: int) -> None:
        self.q2.append(x)
        while self.q1:
            self.q2.append(self.q1.popleft())
        self.q1,self.q2 = self.q2, self.q1

    def pop(self) -> int:
        return self.q1.popleft()
        # self.q1.popleft()
    # push(4), push(8)
    # q1 = [4]
    # q2 = []
    # q1 = [8, 4]
    def top(self) -> int:
        return self.q1[0]

    def empty(self) -> bool:
        return not self.q1 and not self.q2

# 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()

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Back to Day 9 Stack & Queue I · All Algorithm notes