ServiceNow·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

ServiceNow coding round with a string manipulation problem that had two parts. The second part tripped me up more than I expected for something that sounds straightforward.

Questions Asked (1)

Given a string where you can only replace characters (no insertions or deletions), find the minimum number of replacements to satisfy a given condition like making it a palindrome. Then, among all strings that achieve that minimum, return the lexicographically smallest one.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Part one wasn't bad, I got the greedy approach pretty quickly.

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AI HintsAI Generated

Suggested Approach

Use a two-pointer technique to compare characters from both ends, counting mismatches and deciding which character to replace to achieve the lexicographically smallest palindrome. For each mismatch, replace the larger character with the smaller one to minimize the string lexicographically, and handle the middle character if needed. The minimum number of replacements is the number of mismatched pairs.

Pro tip: Clarify that the goal is to minimize replacements first, then lexicographic order; this two-level optimization is key. Also, mention that if the string length is odd, the middle character can be changed to 'a' if it's not already, but only if it doesn't increase the replacement count.

1. Understand the problem and constraints

Restate the problem: only replacements allowed, minimize replacements to make palindrome, then among those, return lexicographically smallest. Confirm that no insertions/deletions are allowed.

2. Identify mismatched pairs with two pointers

Use left and right pointers moving inward. For each pair, if characters differ, it's a mismatch; count it. These mismatches dictate the minimum replacements needed.

3. Determine replacements for lexicographically smallest palindrome

For each mismatched pair, replace the larger character with the smaller one to make them equal, ensuring the resulting string is lexicographically smallest. If characters are equal, consider if they can be reduced to 'a' without increasing replacements (only if both are not 'a' and it doesn't affect mismatch count).

4. Handle the middle character (odd length)

If the string length is odd, the middle character can be changed to 'a' if it's not already, but only if it doesn't increase the total replacements (i.e., it's a free change).

5. Construct and return the result

Build the palindrome by applying the decided replacements, ensuring the minimum replacement count is maintained and the string is lexicographically smallest.

Key Points to Mention

Two-pointer technique for palindrome checking
Greedy choice: replace larger character with smaller for lexicographic minimization
Minimum replacements equals number of mismatched pairs
Handling odd-length middle character for lexicographic improvement
Time complexity O(n) and space complexity O(n) for string manipulation
Edge cases: empty string, single character, all same characters

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