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I went DFS first because it felt more natural to write recursively.
Start by clarifying the board representation and click rules, then outline a BFS solution using a queue to avoid recursion depth issues. Explain the three cases for a clicked cell and how to handle each, then compare BFS vs DFS in terms of complexity and practical trade-offs.
Pro tip: Mention that BFS is generally safer for large boards due to recursion limits, but DFS with an explicit stack can be more memory-efficient for deep, narrow expansions. Also note that marking cells as revealed before enqueueing prevents duplicate processing.
Confirm the board representation (e.g., 'M' for mine, 'E' for empty, digits for adjacent mines) and what happens when clicking a mine or an already revealed cell. Discuss edge cases like clicking outside the board or on a flagged cell.
For the clicked cell: if it's a mine, mark it as 'X' and return. If it's an empty cell with adjacent mines, update it to the count and return. If it's an empty cell with no adjacent mines, reveal it as 'B' and recursively reveal its neighbors.
Use BFS with a queue or DFS with a stack to explore neighbors. For each neighbor, if it's an empty cell with no adjacent mines, mark it as 'B' and add it to the queue/stack; if it has adjacent mines, just update its count.
Time complexity is O(M*N) in the worst case since each cell is processed at most once. Space complexity is O(M*N) for the queue/stack in the worst case, plus the board itself.
BFS uses a queue and explores level by level, avoiding recursion depth issues. DFS uses a stack (or recursion) and may be more memory-efficient for deep expansions but risks stack overflow if recursive. Both have the same asymptotic complexity.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Choose a project that aligns with Pinduoduo's focus on high-scale, high-concurrency systems and clearly articulate your personal ownership. Structure your answer using a narrative arc: context, architecture, your contributions, trade-offs, and measurable impact. Emphasize the 'why' behind decisions and quantify results to show business value.
Pro tip: Quantify impact with metrics that matter to Pinduoduo (e.g., QPS, latency reduction, cost savings, user growth) and be ready to discuss what you would do differently with hindsight—this shows self-awareness and engineering maturity.
Briefly describe the project's goal, your role, team size, and the scale (users, data, traffic). This helps the interviewer understand the environment and your responsibilities.
Outline the system architecture at a high level, focusing on components you directly worked on. Use simple diagrams or analogies if needed, but avoid unnecessary jargon.
Clearly state what you personally designed, built, or improved. Use 'I' statements to distinguish your work from the team's, and highlight technical challenges you solved.
Explain key decisions you made, alternatives considered, and why you chose your approach. Cover technical, business, and team trade-offs (e.g., consistency vs. availability, speed vs. quality).
Share measurable outcomes (e.g., performance improvements, cost reductions, revenue impact) and reflect on what you learned or would do differently. Connect the impact to business goals.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.