Airbnb·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Airbnb SWE interview with a two-part coding problem involving terrain simulation and water physics. The problem sounded manageable at first glance but the water movement rules had enough edge cases to keep things uncomfortable for a while.

Questions Asked (2)

Given an integer array representing terrain heights, write a function to print an ASCII visualization of the terrain using '+' characters, drawn from top to bottom with the base layer always visible.

Algorithms & Data Structures

Author's notes

Part one felt like a warmup and mostly was.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem requirements and edge cases, such as negative heights, zeros, and empty arrays. Then, determine the maximum height to establish the number of rows, and iterate from the top row down to the base, printing '+' for columns where the terrain height is at least the current row level. Finally, test with examples to ensure correctness and discuss time and space complexity.

Pro tip: Mention that the base layer is always visible by ensuring the loop includes the row at height 1 (or 0 if heights can be zero), and handle negative heights by treating them as 0 or clarifying with the interviewer. This shows attention to detail and robustness.

1. Clarify requirements and edge cases

Ask about input constraints: can heights be negative? Is the array empty? What should be printed for zero height? Confirm that the base layer is always visible, meaning at least one row is printed even if all heights are zero.

2. Determine dimensions

Find the maximum height in the array to set the number of rows. The number of columns is the length of the array. If all heights are zero or negative, still print one row (the base).

3. Iterate from top to bottom

Loop from the maximum height down to 1 (or 0 if needed). For each row, iterate through the array and print '+' if the height at that column is >= current row level, otherwise print a space. This builds the visualization from top to bottom.

4. Print and test

Print each row followed by a newline. Test with simple cases like [1,2,3], [0,0,0], and empty array to verify output. Discuss time complexity O(maxHeight * n) and space O(1) excluding output.

Key Points to Mention

Clarify edge cases: negative heights, zero heights, empty array, and whether the base layer should be printed even if all heights are zero.
Determine the maximum height to know how many rows to print, and iterate from top to bottom.
For each row, compare the current row level with each column's height; print '+' if height >= level, else space.
Ensure the base layer is always visible by including the row at level 1 (or 0) in the loop.
Analyze time complexity: O(maxHeight * n) where n is the number of columns, and space complexity O(1) extra space.
Consider optimizing by precomputing or using a more efficient representation if maxHeight is very large, but for ASCII output, the naive approach is acceptable.

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

Extend the terrain problem: implement a function that simulates pouring a given amount of water onto the terrain at a specified column index, then renders the final state using '+' for terrain and 'W' for settled water. Water moves left or right based on effective height comparisons.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This is where things got real.

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AI HintsAI Generated

Suggested Approach

Model the terrain as an array of heights and simulate water flow by repeatedly finding the lowest adjacent column that is lower than the current water level, moving water there until it settles. Use a priority queue or BFS to efficiently propagate water to local minima, then render the final state by overlaying water units on the terrain grid.

Pro tip: Clarify assumptions about water behavior (e.g., does it flow off edges? can it evaporate?) and discuss trade-offs between simulation approaches (e.g., iterative vs. priority queue) to demonstrate engineering maturity.

1. Clarify requirements and constraints

Ask about terrain representation, water flow rules (e.g., does water flow off edges? can it pool indefinitely?), and output format. Confirm the column index is 0-based or 1-based.

2. Design the simulation algorithm

Choose an approach: iterative water movement using a queue or priority queue to always process the lowest point, or a union-find based method to find basins. Consider time and space complexity.

3. Implement water placement and flow

Place the given amount of water at the specified column. Simulate flow by moving water to adjacent lower columns (considering effective height = terrain height + water level) until no more movement is possible.

4. Render the final state

Create a 2D grid where each column's terrain is represented by '+' characters up to its height, and water is represented by 'W' above the terrain up to the water level. Ensure proper alignment and spacing.

5. Test and validate

Test with edge cases: no water, water exceeding capacity, flat terrain, steep slopes, and multiple basins. Verify that water settles correctly and rendering matches expectations.

Key Points to Mention

Effective height concept: water level plus terrain height determines flow direction.
Use of priority queue (min-heap) to efficiently find the lowest adjacent column for water movement.
Handling of water that cannot settle (e.g., flows off edges) and how it affects the final state.
Time and space complexity analysis of the chosen algorithm.
Rendering details: how to represent water above terrain and ensure correct column alignment.
Trade-offs between different simulation strategies (e.g., BFS vs. iterative relaxation).

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