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Geico·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Geico SWE interview that was basically a deep dive into one string manipulation problem. They wanted more than just a working solution, they pushed on the reasoning behind the approach and the complexity tradeoffs, which I wasn't fully prepared for.

Questions Asked (1)

Q1

Given a string of digits, construct the largest possible palindromic number using some or all of those digits. The result cannot have leading zeros unless it's just '0' itself.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Started with brute force because I figured they'd want to see my thinking.

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AI HintsAI Generated

Suggested Approach

Count the frequency of each digit, then greedily build the largest palindrome by placing the largest possible digits at the ends, using the largest remaining digit as the center if any odd count exists. Handle edge cases like all zeros and leading zeros by ensuring the first digit is non-zero unless the result is '0'.

Pro tip: Clarify constraints upfront (e.g., input size, whether all digits must be used) and discuss trade-offs between time and space complexity; this shows you think about real-world scenarios and not just the algorithm.

1. Clarify requirements and edge cases

Ask about input size, whether all digits must be used, and how to handle leading zeros. Confirm that the result should be the largest possible palindromic number.

2. Count digit frequencies

Use an array of size 10 to count occurrences of each digit. This allows O(n) time and O(1) space for the counting step.

3. Construct the first half

Iterate from digit 9 down to 0, appending half of each digit's count to the left half. This ensures the largest possible digits are placed at the most significant positions.

4. Form the palindrome

Combine the left half, a middle digit (the largest digit with an odd count, if any), and the reverse of the left half. Handle the all-zeros case separately.

5. Validate and handle edge cases

Check for leading zeros: if the result starts with '0' and length > 1, it's invalid; return '0' if all digits are zero. Also consider if no palindrome can be formed (e.g., empty input).

Key Points to Mention

  • Time complexity: O(n) for counting and O(n) for building the result, where n is the number of digits.
  • Space complexity: O(1) for the frequency array, O(n) for the output string.
  • Greedy strategy: always pick the largest available digit for the ends to maximize the number.
  • Handling odd counts: at most one digit can have an odd count in a palindrome; use the largest such digit as the center.
  • Edge cases: all zeros, single digit, no possible palindrome (e.g., empty string), and leading zeros.
  • Trade-offs: using a frequency array vs sorting; sorting would be O(n log n) but simpler, while counting is O(n) and more efficient.

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