Katkat

Debug and optimize a card-drawing strategy

MetaMachine Learning Engineer · Onsite

Last updated

This problem tests your ability to simulate card-drawing probabilities, debug flawed logic in iterative selection strategies, and optimize performance for repeated simulations.

Problem Statement

You are given a deck of $N$ unique cards, each with an associated value. You are tasked with implementing a function select_optimal_hand(cards, k, strategy) that draws $k$ cards to maximize the sum of their values, given a specific, potentially flawed, strategy.

The Flawed Strategy

The current implementation iterates through the deck once. For each card, it calculates the probability of drawing a 'better' card in the remaining deck. If the probability is below a threshold $P$, it keeps the card; otherwise, it discards it.

Your task is to fix the function so that it correctly handles the state of the deck (which changes as cards are drawn) and performs efficiently for large decks.

Function Signature

def select_optimal_hand(cards: List[int], k: int, threshold: float) -> int:
    # cards: List of integers representing card values
    # k: Number of cards to draw
    # threshold: Probability threshold (0.0 to 1.0)
    pass

Requirements

  1. Maintain a running state of the remaining deck.
  2. Ensure the selection logic correctly accounts for the diminishing pool of cards after each draw.
  3. Return the maximum possible sum if the strategy is followed perfectly until $k$ cards are selected.

Example

Input: cards = [10, 5, 8, 2, 9], k = 2, threshold = 0.5

  • Card 10: 4 cards remaining, 0 better than 10. Prob = 0.0. Keep.
  • Card 5: 3 cards remaining, 2 better than 5 (8, 9). Prob = 2/3 (0.66). Discard.
  • Resulting logic should track state changes correctly.

Constraints

  • $N$ up to 10,000.
  • $k < N$.
  • Time complexity should be better than $O(N^2)$.

Hints

Hint 1

Instead of recalculating the number of 'better' cards from scratch each iteration, use a sorted frequency array or a Fenwick tree (Binary Indexed Tree) to count better cards in logarithmic time.

Hint 2

The probability of drawing a better card is defined as (count of remaining cards > current) / (total remaining cards).

Hint 3

Pre-sorting the deck or using a min-heap to keep track of discarded values might help simplify the conditional logic.

Follow-up questions

  • How would you modify this algorithm if the deck were shuffled after each draw?
  • If the threshold $P$ was unknown, how would you use Monte Carlo simulations to find an optimal $P$ that maximizes the total sum for a given $k$?
  • How does the time complexity change if we need to support dynamic updates (adding/removing cards) to the deck during the selection process?