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Two Sum · Medium

Two Sum Medium

Problem Discussion

Given an array of integers nums and an integer target, return indices of the two numbers that add up to target.

You may assume each input has exactly one solution, and you may not use the same element twice.

Example

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: nums[0] + nums[1] = 9

Constraints

2 ≤ nums.length ≤ 10⁴

solution.pytests

1def twoSum(nums, target):
  seen = {}
  for i, n in enumerate(nums):
      diff = target - n
      if diff in seen:
          return [seen[diff], i]
      seen[n] = i

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Multi-Level

Progressive challenges that build on each level

OA-style problems where each level adds complexity. Start with the basics, unlock harder levels as you go — just like the real online assessments.

In-Memory Database · Hard

2 Transactions

Level 2 / 4

Add transaction support

Implement BEGIN, COMMIT, and ROLLBACK operations

Level 1 · Basic GET/SET/DELETEDone

2Level 2 · TransactionsCurrent

3Level 3 · Nested transactionsLocked

4Level 4 · Expiry & persistenceLocked

solution.pytest_db.pyREADME.md

class Database: def begin(self): # Start a new transaction pass def commit(self): pass def rollback(self): pass

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Word Guessing Game · Medium

Files

PYgame.py

PYtests.pyread-only

Part 1 Debug

game.py

1class WordGame:
  def __init__(self, word):
      self.secret_word = word.lower()
      self.guessed = set()
      self.wrong = 0
6
  def guess(self, letter):
      letter = letter.lower()
      self.guessed.add(letter)
      if letter in self.secret_word:
          return "correct"
      self.wrong += 1
      return "wrong"

Console Test Results AI Submissions

3 tests are failing. The losing condition seems off — can you check?

Thinking...

Looking at get_status(). The condition wrong_guesses > max_wrong uses > but should be >=. Also get_remaining_guesses has an off-by-one.

I see two bugs in game.py:

1. get_status() line 125 — should be >= not >:

if self.wrong_guesses >= self.max_wrong:
    return "lost"

2. get_remaining_guesses() has an off-by-one:

return self.max_wrong - self.wrong_guesses

Fix both and the tests should pass.

Why >= matters? Third bug?

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Design DoorDash

2 Architecture

How To Your Answer Feedback

Draw the architectureSketch the main components: mobile/web clients, API gateway, microservices, databases, and message queues.

Show data flowUse arrows to show how an order flows from the customer through the system. Label protocols (HTTP, gRPC).

Consider scalePeak dinner hours drive 10x traffic. Where do you add caching? What needs async processing?

Your work is on the canvas

Draw your architecture diagram on the right

No feedback yet

Click Check to get AI evaluation

85Good

Strengths:

Clean microservice separation. Good use of event-driven architecture with Kafka for async workflows. Redis caching for menu/restaurant data is solid.

Areas to improve:

Consider a dedicated delivery matching service. Add a payment service between order and restaurant. WebSocket for real-time order tracking.

50+

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Title

Rate Limiter

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Medium

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class RateLimiter: def __init__(self, limit): pass

Prepare for any process

Different companies interview differently. Vahalla covers the formats you'll actually encounter.

Big Tech Interviews

Algorithm-heavy rounds with follow-up questions and complexity analysis. Multi-level problems that build in difficulty.

AlgorithmsData StructuresMulti-Level

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AI-EnabledSystem DesignProduction-Style

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