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DoorDash·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

DoorDash software engineer interview with a string manipulation problem. Pretty standard coding round, nothing too wild, but the problem has some edge cases that can trip you up if you're not careful.

Questions Asked (1)

Q1

Given a target string and a list of strings, return all entries from the list that could match the target string by swapping exactly one pair of characters.

Algorithms & Data Structures
Author's notes

The base case where both strings are already equal tripped me up a bit.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints (e.g., string lengths, character set, case sensitivity) and edge cases (e.g., duplicates, empty strings). Then propose an efficient algorithm: for each string in the list, check if it can be transformed into the target by exactly one swap, using a linear scan to identify mismatched positions and verifying the swap condition. Discuss time and space complexity, and consider optimizations like early termination or grouping by length.

Pro tip: Mention that you would first filter out strings with different lengths or character frequencies, as they can never match by a single swap. This shows you think about pruning and efficiency before diving into code.

1. Clarify requirements and edge cases

Ask about input constraints (string lengths, character set, case sensitivity), output order, and handling of duplicates. Confirm that 'exactly one pair' means swapping two distinct characters (possibly equal? clarify if swapping identical characters counts).

2. Design the matching condition

For a candidate string to match the target by one swap, they must have the same length and same character multiset. Then, find the positions where they differ; there must be exactly two mismatches, and swapping those characters in the candidate must yield the target.

3. Implement an efficient check

Iterate through each string in the list. First, check length equality. Then, scan both strings simultaneously to collect mismatched indices. If exactly two mismatches are found and the characters cross-match (candidate[i] == target[j] and candidate[j] == target[i]), the string is a match.

4. Analyze complexity and optimize

The naive approach is O(N * L) where N is the number of strings and L is the length. Discuss potential optimizations: pre-grouping by length, using a hash map of character counts, or early termination when more than two mismatches are found.

5. Test with examples and edge cases

Walk through examples: target 'abcd', list ['abdc', 'abcd', 'abca', 'abcde']. Verify that 'abdc' matches (swap c and d), 'abcd' does not (zero swaps), 'abca' does not (different multiset), 'abcde' does not (different length). Also test empty strings and strings with repeated characters.

Key Points to Mention

  • Length and character frequency must match for a single swap to be possible.
  • Exactly two mismatched positions are required; if there are zero or more than two, it's not a single swap.
  • The swap condition: candidate[i] == target[j] and candidate[j] == target[i].
  • Time complexity O(N * L) and space complexity O(1) per string (excluding output).
  • Handling duplicates: if multiple identical strings match, include all occurrences.
  • Edge cases: empty strings, strings with all identical characters, and swapping identical characters (clarify if allowed).

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