Google·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

Google SWE coding round, one algorithmic problem on string transformations. Pretty clean problem once you see the trick, but I spent way too long second-guessing my pointer logic.

Questions Asked (1)

Given two strings of equal length made up of 'L', 'R', and '_', determine whether you can transform the first string into the second by sliding L's left and R's right into adjacent blank spaces.

Algorithms & Data Structures

Author's notes

The key insight is that you ignore the blanks and just match up the non-blank characters in order.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem constraints and edge cases. Then, propose an efficient algorithm that checks invariants: the sequence of non-underscore characters must match, and each L must not move right, each R must not move left. Finally, analyze time and space complexity and discuss potential optimizations.

Pro tip: Mention that this problem is similar to checking if two strings are anagrams with positional constraints, and that a two-pointer approach can solve it in O(n) time. Also, emphasize the importance of handling edge cases like multiple underscores and no moves possible.

1. Understand the problem

Restate the problem in your own words and ask clarifying questions about constraints, input size, and expected output format.

2. Identify invariants

Determine that the relative order of non-underscore characters must be preserved, and that L's can only move left while R's can only move right.

3. Design an algorithm

Use a two-pointer technique to compare the strings while skipping underscores, ensuring that L's don't move right and R's don't move left.

4. Analyze complexity

State that the algorithm runs in O(n) time and O(1) extra space, which is optimal.

5. Test with examples

Walk through a few test cases, including edge cases like all underscores, no moves needed, and impossible transformations.

Key Points to Mention

Preservation of the relative order of non-underscore characters.
Directional constraints: L moves left, R moves right.
Two-pointer approach for O(n) time complexity.
Handling of edge cases: empty strings, all underscores, no possible moves.
Comparison of positions: for L, start index must be >= target index; for R, start index must be <= target index.
Space complexity: O(1) extra space.

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