Oracle·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Oracle SWE interview with a robot grid simulation problem. The question sounds deceptively simple but the reasoning behind the answer is what they're really after, not just the code.

Questions Asked (1)

A robot starts at (0,0) facing North on an infinite grid and executes a sequence of moves ('G' to go forward, 'L' to turn left, 'R' to turn right). The sequence repeats forever. Write a function that returns true if the robot stays within some bounded circle, and justify your reasoning including edge cases.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I got the base cases fast enough: if after one cycle the robot is back at origin, it loops forever, bounded.

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AI HintsAI Generated

Suggested Approach

Simulate the robot's movement for a bounded number of steps, tracking its position and direction, and detect if it returns to a previously seen state (position and direction) or if its position exceeds a safe bound. If a cycle is detected, the robot is bounded; otherwise, it will drift away. Justify using the pigeonhole principle and discuss edge cases like empty sequence or sequences that only rotate.

Pro tip: Emphasize that the state space is finite (position modulo something and direction), so cycle detection is both necessary and sufficient; mention that you can optimize by simulating at most 4 * (max displacement per cycle) steps, but a simple visited set works for clarity.

1. Understand the problem and state representation

Clarify that the robot's state is defined by its position (x, y) and direction (N, E, S, W). The sequence repeats, so the robot's behavior is periodic if it ever revisits a state.

2. Choose a detection strategy

Decide to simulate the robot's moves step by step, recording each state in a set. If a state repeats, the robot is in a cycle and thus bounded; if the position exceeds a large bound (e.g., 4 times the length of the sequence), it's unbounded.

3. Implement simulation with cycle detection

Iterate through the sequence, updating position and direction. After each move, check if the new state is already in the set. If yes, return true; if the number of steps exceeds a safe limit, return false.

4. Justify correctness and complexity

Explain that the state space is finite (position bounded by displacement per cycle, direction 4 ways), so if the robot doesn't repeat a state within a certain number of steps, it must be moving away. Time complexity is O(n) where n is the sequence length times a constant factor.

5. Discuss edge cases and trade-offs

Cover cases like empty sequence, sequences that only rotate (robot stays at origin), sequences that move in a straight line (unbounded), and sequences that form a loop. Mention that using a set of states is simple but uses O(n) space; alternatively, one can simulate 4 cycles and check if displacement is zero.

Key Points to Mention

Finite state space: position modulo displacement and direction (4 possibilities) ensures cycle detection works.
Pigeonhole principle: if the robot doesn't repeat a state within a bounded number of steps, it must be unbounded.
Simulation bound: simulate at most 4 * (length of sequence) steps or until a state repeats.
Edge cases: empty sequence, only turns (L/R), straight line (G only), and sequences that return to origin with same direction.
Time and space complexity: O(n) time and O(n) space for visited set; can optimize space by simulating 4 cycles and checking net displacement.
Justification: if after 4 cycles the robot returns to origin with same direction, it's bounded; otherwise, it drifts away.

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