Walmart Labs·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Walmart Labs SWE interview, got a simulation-style array problem that looks deceptively simple but needs a real approach to avoid TLE on large inputs. No frills, just the one problem and then done.

Questions Asked (1)

You have an array of plants with pesticide levels. Each day, any plant whose pesticide level is strictly greater than the plant directly to its left gets removed simultaneously. How many days does this process take before no more plants die?

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just simulate it day by day with a loop, which works fine for small inputs but blows up at n=1e5.

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AI HintsAI Generated

Suggested Approach

Clarify the problem and constraints, then propose an efficient O(n) solution using a monotonic stack to track the number of days each plant survives. Walk through a concrete example to validate the approach, and discuss time/space complexity and edge cases.

Pro tip: Emphasize that the process is equivalent to finding the maximum number of consecutive 'drops' in the array, and that a stack-based solution avoids simulating each day, which would be O(n^2) in the worst case.

1. Understand the problem

Restate the problem in your own words and confirm details: plants are removed simultaneously each day if their pesticide level is strictly greater than the plant to their left. The goal is to find the number of days until no more removals occur.

2. Explore naive and optimized approaches

Discuss a brute-force simulation that scans the array each day, noting its O(n^2) worst-case time. Then introduce the idea of using a monotonic stack to compute the survival days for each plant in a single pass.

3. Design the stack-based algorithm

Iterate through the array, maintaining a stack of pairs (pesticide level, days survived). For each plant, pop elements greater than or equal to the current level, tracking the maximum days among popped elements. The current plant's survival days is that maximum plus one (if any element was popped), otherwise zero. Push the current plant and update the global maximum days.

4. Validate with examples and edge cases

Test the algorithm on small arrays (e.g., [3,2,1], [1,2,3], [5,3,4,2,1]) to ensure correctness. Consider edge cases: strictly increasing array (all plants except first die on day 1), strictly decreasing array (no plants die), and arrays with duplicates.

5. Analyze complexity and discuss trade-offs

State that the stack approach runs in O(n) time and O(n) space, which is optimal. Contrast with the O(n^2) simulation, and mention that if the array is very large, the stack method is preferred despite the extra space.

Key Points to Mention

The process is simultaneous, so a plant's removal depends only on the state at the beginning of the day.
A plant dies on day d if there is a chain of d consecutive plants to its left with strictly decreasing pesticide levels ending at a plant with a lower level than the current plant.
Monotonic stack efficiently computes the number of days each plant survives by maintaining a decreasing sequence of pesticide levels.
The answer is the maximum survival days among all plants.
Time complexity: O(n) with a single pass; space complexity: O(n) for the stack.
Edge cases: strictly increasing array (answer 1), strictly decreasing array (answer 0), and arrays with equal adjacent elements (no removal).

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