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Apple·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Phone screen for a software engineer role at Apple. Just one coding question, nothing else to report.

Questions Asked (1)

Q1

Given a 2D board of characters and a list of words, find all words that exist in the board. A word can be constructed from letters of sequentially adjacent cells (horizontally or vertically neighboring), and the same cell may not be used more than once per word.

Algorithms & Data Structures
Author's notes

Classic trie plus DFS backtracking problem.

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AI HintsAI Generated

Suggested Approach

Model the board as a graph and use DFS with backtracking to explore all possible paths for each word, but optimize by building a trie of the word list to prune searches early. Start from each cell and traverse adjacent cells, marking visited cells to avoid reuse, and collect words that match trie prefixes.

Pro tip: Mention that you can further optimize by removing words that are prefixes of others or by sorting words by length, and discuss trade-offs between time and space complexity. Also, highlight the importance of early termination when no words remain to be found.

1. Clarify requirements and constraints

Ask about board dimensions, word list size, character set, and whether words can be reused. Confirm that each cell can be used at most once per word and that adjacency is 4-directional.

2. Choose data structures

Decide to use a trie to store the word list for efficient prefix matching, and a visited matrix or in-place marking to track cell usage during DFS.

3. Design the algorithm

Outline a DFS backtracking approach: for each cell, if its character matches a trie child, recurse to adjacent cells, marking visited, and if a word end is reached, add to results. Prune branches when no trie node exists.

4. Analyze complexity and optimizations

Discuss time complexity O(M*N*4^L) worst-case but improved by trie pruning, and space O(total characters in words). Mention optimizations like removing found words from trie to avoid duplicates.

5. Test with edge cases

Walk through examples: single cell, no words, words longer than board, overlapping paths. Ensure code handles empty board or word list.

Key Points to Mention

  • Use a trie to store the dictionary for efficient prefix pruning.
  • Implement DFS with backtracking, marking cells as visited and unmarking on backtrack.
  • Avoid revisiting the same cell within a single word by using a visited set or modifying the board temporarily.
  • Optimize by removing words from the trie once found to prevent duplicate results and reduce search space.
  • Consider time and space complexity: O(M*N*4^L) worst-case, but trie reduces practical runtime.
  • Handle edge cases: empty board, empty word list, words longer than board dimensions, and single-character words.

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