Meta·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Meta SWE coding round, one question the whole time. The robot bounded in circle problem, which sounds straightforward until you actually have to reason through all the edge cases on the fly.

Questions Asked (1)

Given a sequence of movement instructions for a robot starting at the origin and facing north, determine whether the robot will remain bounded within some finite region if the instructions are repeated indefinitely. The solution must use constant space.

Algorithms & Data Structures

Author's notes

I knew the key insight was about direction after one full cycle, not just position.

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AI HintsAI Generated

Suggested Approach

Simulate one pass of the instructions while tracking the robot's final position and orientation. If the robot returns to the origin, it is bounded; if it faces north again but is not at the origin, it is unbounded; otherwise, it will eventually cycle and remain bounded. This uses constant space by only storing the final state.

Pro tip: Clearly explain the three cases and why they cover all possibilities, emphasizing that the robot's path repeats every 1 or 4 cycles, so checking one pass is sufficient.

1. Understand the problem

Clarify that the robot starts at (0,0) facing north, and the instructions are repeated indefinitely. We need to determine if the robot's path stays within a finite region.

2. Simulate one pass

Process the instructions once, updating the robot's position (x, y) and direction (N, E, S, W) accordingly. Use constant space by only storing these variables.

3. Analyze the final state

After one pass, check: (a) if the robot is back at the origin, it will loop and remain bounded; (b) if it faces north but is not at the origin, it will drift away and be unbounded; (c) otherwise, it will cycle every 4 passes and remain bounded.

4. Return the result

Based on the analysis, return true if bounded, false if unbounded. Explain why the three cases cover all possibilities.

Key Points to Mention

Constant space complexity: only store position and direction, no extra data structures.
The robot's orientation after one pass determines the cycle length (1 or 4).
If the robot returns to the origin, it is bounded regardless of orientation.
If the robot faces north but is not at the origin, it will move infinitely in one direction.
In all other cases, the robot's path repeats every 4 passes, forming a closed loop.
Time complexity is O(n) where n is the number of instructions.

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