LinkedIn·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

LinkedIn SWE coding round, one problem the whole time. It's a phone keypad / dictionary lookup thing, basically LeetCode 17 but with a word list constraint. Felt manageable once I figured out the pruning angle.

Questions Asked (1)

Given a string of digits (2-9), where each digit maps to letters like a phone keypad, and a dictionary of valid words, return all letter combinations of the digit string that appear in the dictionary.

Algorithms & Data Structures

Author's notes

I started with the naive backtracking approach, generate every combination then filter against a set.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints, then discuss a backtracking approach that generates all possible letter combinations and checks each against the dictionary. Optimize by using a trie for the dictionary to prune invalid prefixes early, and analyze time and space complexity.

Pro tip: Mention that you can preprocess the dictionary into a trie and then perform DFS on the digit string, pruning branches that don't match any word prefix. This shows you think about efficiency and real-world scalability.

1. Clarify requirements and constraints

Ask about input size, dictionary size, whether the output should be sorted, and if duplicates are possible. Confirm the mapping of digits to letters.

2. Discuss brute-force approach

Explain generating all possible letter combinations using backtracking and then filtering by dictionary lookup. Mention time complexity O(4^n * m) where n is digit length and m is average word length.

3. Optimize with trie

Propose building a trie from the dictionary and performing DFS on the digit string, only continuing down paths that are prefixes in the trie. This reduces unnecessary combinations.

4. Analyze complexity and edge cases

Discuss time and space complexity of the optimized solution, and cover edge cases like empty input, no matches, and digits mapping to multiple letters.

5. Code and test

Write clean code for the chosen approach, and walk through a test case to verify correctness.

Key Points to Mention

Backtracking/DFS for generating combinations
Trie data structure for efficient prefix matching
Time and space complexity analysis
Handling edge cases (empty string, no matches)
Digit-to-letter mapping (like phone keypad)
Pruning invalid paths early to optimize

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