Palantir Coding Interview: War Card Game Simulation

Problem summary

You must implement a simulator for one round of an extended multiplayer version of the card game “War”. Each player has a deck modeled as a queue (top = pop from left, bottom = append to right). In a single round, every active player places a face-up card into a shared common pile; the highest unique face-up card wins the pile. Ties among top face-up cards trigger a tie-resolution loop where only the tied players add up to three face-down cards followed by a new face-up card, repeating until one winner emerges or all tied players are eliminated.

What you'll be asked to do

You should provide a clear function signature (for example, simulate_round(decks: List[Deque[int]]) -> (updated_decks, winner_index)) and implement exactly one round. Be explicit about: how player indices map to decks, how the common pile preserves play order, how and when players are eliminated, and what happens when a player cannot supply the required number of cards.

Flow / stages

• Initial face-up draw by all active players
• Compare face-up cards; if unique max → winner collects pile in played order
• If tie → tie-resolution among tied subset (up to 3 face-down + 1 face-up per tied player)
• If a player runs out of cards they place all remaining cards; empty decks → eliminated
• If all tied players are eliminated → no winner; pile removed from play

Skill signals the interviewer is evaluating

You should demonstrate correct queue/deque usage, careful ordering of the common pile, robust handling of edge cases (insufficient cards, simultaneous elimination), and clear complexity reasoning. Bonus: detect and reason about long-running games or cycle detection if asked to simulate until a single remaining player.