Sign in
Skip to solution
mediumDSA

How do you implement a queue using two stacks?

1.1k views
01

Understand the problem

Explain queue from stacks.

stackqueue
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

Keep an in stack and an out stack. Push to in. To dequeue/peek, if out is empty, pop everything from in into out (reversing order), then pop out. Each element moves at most twice — amortized O(1).

Solution ready — 2 min read

Classified // press E to declassify

04

Read the code

Queue via in/out stacks
Run Playground
class MyQueue:
    def __init__(self):
        self.in_stack = []
        self.out_stack = []

    def enqueue(self, x):
        self.in_stack.append(x)

    def dequeue(self):
        self._shift()
        return self.out_stack.pop()

    def peek(self):
        self._shift()
        return self.out_stack[-1]

    def _shift(self):
        if not self.out_stack:
            while self.in_stack:
                self.out_stack.append(self.in_stack.pop())


# --- demo ---
q = MyQueue()
q.enqueue(1); q.enqueue(2); q.enqueue(3)
print(q.dequeue())   # 1
print(q.dequeue())   # 2
q.enqueue(4)
print(q.dequeue())   # 3
05

Join the discussion

Discussion (0)

Sign in to join the discussion.

No responses yet. Be the first to share what you think.

Transmission complete // awaiting log

KEEP THE
STREAK ALIVE.

Dossier 36 of 127 decoded in the Data Structures & Algorithms track. One more won't hurt.

Back to track