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Google·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Went through a Google SWE coding round with a robot movement simulation problem. Pretty clean question, nothing too wild, but the details matter more than they look.

Questions Asked (1)

Q1

Given a starting position and a sequence of movement instructions (U, D, L, R), determine whether a robot ends up back at its starting position after executing all the moves.

Algorithms & Data Structures
Author's notes

Looks trivial until you second-guess yourself.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints (e.g., input format, move set) and then propose a simple simulation using two counters for vertical and horizontal displacement. Explain that the robot returns to start if and only if both counters are zero after processing all moves, and analyze the time and space complexity.

Pro tip: Mention that you can early-exit if the number of moves is odd, since each move changes exactly one coordinate by ±1, making a return to origin impossible. This shows attention to edge cases and optimization.

1. Clarify the problem

Ask about input format (string or array), move set (only U, D, L, R), and whether the starting position is always (0,0). Confirm that moves are unit steps.

2. Choose a representation

Use two integer variables (x, y) to track the robot's position, initialized to (0,0). Alternatively, use a hash map to count moves, but counters are simpler.

3. Simulate the moves

Iterate through each move and update x or y accordingly: U increases y, D decreases y, R increases x, L decreases x.

4. Check the final position

After processing all moves, check if x == 0 and y == 0. If so, return true; otherwise, return false.

5. Analyze complexity and edge cases

State that time complexity is O(n) and space is O(1). Mention edge cases: empty string, odd number of moves, and invalid characters.

Key Points to Mention

  • Simulation with two counters for x and y coordinates.
  • Time complexity O(n) and space complexity O(1).
  • Early exit if the number of moves is odd.
  • Handling invalid characters or unexpected input.
  • Alternative approach: count U vs D and L vs R, and check if counts are equal.
  • Clarifying assumptions about starting position and move set.

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