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SoFi·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Interviewed for a full-stack role at SoFi and got hit with a tournament simulation problem that looked manageable until I actually tried to code it up under pressure. The brute force path is obvious but they clearly wanted the greedy single-pass solution.

Questions Asked (1)

Q1

There are N+1 players in a queue. Each player from index 1 onward has a fixed hand in rock-paper-scissors. You are player 0 and can freely change your hand between matches. Matches go front-to-back, winner stays, but a tie eliminates both players and the next pair starts fresh. You need to defeat whoever reaches you and every player after them. Return the minimum number of hand changes required.

Algorithms & Data Structures
Author's notes

Jumped straight to simulating all three starting choices and got something working, but it was O(N^2) and they asked me to do better.

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AI HintsAI Generated

Suggested Approach

First, clarify the rules and edge cases (e.g., tie elimination, winner stays) to ensure correct understanding. Then, model the process as a state machine where the current opponent is determined by simulating matches, and use dynamic programming to compute the minimum hand changes needed to defeat the current opponent and all subsequent players. Finally, discuss time and space complexity and potential optimizations.

Pro tip: Mention that you would test with small cases and edge cases (e.g., all ties, no ties) to validate the DP transitions, and emphasize that the key is to track the minimum changes up to each player while considering the current hand.

1. Clarify rules and edge cases

Ask questions to confirm: ties eliminate both players and the next pair starts fresh; winner stays and faces the next player; you can change hand only between matches; you need to defeat the player who reaches you and all after them.

2. Define state and transitions

Define DP state as the minimum hand changes needed to defeat the current opponent and all subsequent players, given the current hand. Simulate the queue to determine who the current opponent is based on previous outcomes.

3. Derive recurrence relation

For each possible hand (rock, paper, scissors), compute the outcome against the current opponent. If you win, you stay and face the next player; if you lose, you are eliminated and the next player faces the opponent. Use DP to minimize changes.

4. Implement and optimize

Implement the DP iteratively from the end of the queue backwards, or use memoization. Optimize by noting that only three hands are possible, so state space is small. Discuss time and space complexity.

5. Test and validate

Walk through small examples (e.g., N=1, N=2) and edge cases (e.g., all ties, no ties) to ensure the DP correctly handles tie eliminations and winner progression.

Key Points to Mention

  • Dynamic programming with state representing the current hand and the current opponent index.
  • Simulation of the queue to determine the current opponent based on previous match outcomes.
  • Handling of tie cases: both players eliminated, next pair starts fresh.
  • Minimizing hand changes: only change when necessary, and consider all three hands at each step.
  • Time and space complexity: O(N) time and O(1) space if optimized, since only three hands.
  • Edge cases: all ties, no ties, N=0, and ensuring the last player is defeated.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.