Meta — Senior Software Engineer

Executive summary

You completed both problems correctly and demonstrated solid fundamentals: good clarifying questions, clear narration of your approach, and accurate complexity analysis. The main gap is in solution exploration depth — you jumped to the optimal approach quickly on Q1 without fully articulating the trade-off, and on Q2 you needed interviewer nudging before landing on the iterative BFS approach and correctly structuring the queue payload. At a staff level, driving those design decisions proactively and without prompting is the expectation.

Strengths

• Active edge-case clarification before coding

  On both problems you asked targeted clarifying questions before writing a single line of code. On Q1 you confirmed character set, special characters, and whether numbers/spaces were possible. On Q2 you confirmed digit-only node values and the empty-root edge case. This is exactly the right habit.

  • first thing I want to ask is is everything's going to be uh just lowercase letters right
  • would it be uh would the values of those nodes be from zero to nine or um is it also could also be like 12 exist as a node value

• Accurate complexity analysis throughout

  You correctly identified O(n) time and O(n) space for both the hashmap Q1 solution and the BFS Q2 solution, and you justified the space bound on Q2 by reasoning about queue occupancy rather than just stating it.

  • it would be basically whatever the input size is um so it would be I believe it would be o of n for that

• Thorough, narrated test-case walkthrough

  You walked through both solutions with concrete inputs step by step, tracking the queue state, digit accumulation, and final sum explicitly. This demonstrates you trust your own solution enough to verify it rather than declare it correct and stop.

  • so we then pop it so again this this gets removed um and our node is one current current sum is one
  • 12 * 10 which would be 120 + 4 124 so 4 124 um continue

• Caught own bug mid-walkthrough without prompting

  While tracing Q2 you noticed the right-child append line still referenced node.left.val instead of node.right.val and flagged it yourself before the interviewer needed to point it out. Self-correction mid-trace is a positive signal.

  • q.append node.write current sum times 10 plus no. left. Val um

Improvements

• Articulate trade-offs between approaches before choosing

  On Q1 you briefly mentioned the naïve O(n²) approach, then immediately pivoted to the hashmap solution without explaining why the hashmap is better beyond 'there's definitely better ways'. At staff level the expectation is to name the trade-off axis explicitly: runtime, memory overhead, code complexity, readability. A one-sentence comparison ('the hashmap trades O(n) extra space for a single pass instead of a nested loop, which is the right call when n is large') would close this gap.

  Try this next time

  In your next session, after proposing any second approach, force yourself to say one sentence that completes this template: 'I prefer approach B over approach A because [trade-off axis] even though it costs [downside].' Rehearse this phrase until it becomes automatic.

  • while that's the nice solution there's definitely better ways to do this um we can just use it by um by just using a hashmap

• Drive the iterative-vs-recursive decision yourself on tree problems

  On Q2 you proposed recursion first and only pivoted to BFS after the interviewer explicitly raised the stack-overflow concern. You should proactively mention call-stack depth as a risk whenever you reach for DFS recursion on a tree, especially at staff level where production concerns are expected to be self-surfaced.

  Try this next time

  Every time you propose a recursive tree solution, immediately add: 'One thing to flag — recursion depth is bounded by tree height, so if the tree is skewed or very deep we'd want an iterative version using an explicit stack or queue.' Practice saying this sentence after every recursive tree proposal in your next five mock sessions.

  • let's kind of assume that the tree can be really big and trying to recurse through it might um we might have memory issues yeah what's the alternative approach

• Pre-decide the queue payload structure before writing the BFS loop

  You figured out the (node, current_sum) tuple design while narrating mid-code. That reasoning is correct but it came out hesitantly and only after you started writing. At staff level, state the queue payload design as a deliberate design decision before the first line of the loop body.

  Try this next time

  Before writing any BFS loop, write a one-line comment in your editor such as `# queue stores (node, running_digit_number)` and say it out loud. This forces you to commit to the data structure design explicitly and signals clear intent to the interviewer.

• Tighten complexity framing: use two variables when appropriate

  On Q2 you said time complexity is 'O of whatever the diameter of the tree is' before correcting to O(n). The diameter comment introduced unnecessary ambiguity. More broadly, when a problem has two independent inputs (like Q1's input string and order string), use separate variable names (e.g. O(n + k)) rather than collapsing everything to O(n).

  Try this next time

  After every complexity statement, ask yourself: 'Are there two independent inputs here? If so, do they both appear in my bound?' If yes, name them separately. Spend five minutes before your next mock reviewing how to express complexity for two-input string problems.

  • for time complexity it would be o of n because we're still iterating through well o of whatever the um diameter of the tree is

Communication signals

Coding — round detail