Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

Meta SWE coding round, one question about custom alphabet ordering. Pretty focused session, no fluff.

Questions Asked (1)

You're given a custom alphabet ordering and an array of strings. Write a function that checks whether the array is sorted lexicographically according to that custom ordering, returning true or false.

Algorithms & Data Structures

Author's notes

My first instinct was to build a lookup map from character to its rank in the custom alphabet, then compare adjacent strings pairwise.

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AI HintsAI Generated

Suggested Approach

First, map each character in the custom alphabet to its index for O(1) lookups. Then, iterate through the array and compare each adjacent pair of strings character by character using the custom order. If all pairs are in non-decreasing order, return true; otherwise, return false.

Pro tip: Clarify edge cases upfront, such as strings of different lengths and the scenario where one string is a prefix of another. Also, mention that the solution should handle large inputs efficiently by avoiding unnecessary conversions.

1. Clarify the problem and edge cases

Ask questions to confirm the definition of lexicographical order with a custom alphabet, including how to handle strings of different lengths and prefix cases. Discuss constraints like input size and character set.

2. Preprocess the custom alphabet

Create a hash map or array to store the index of each character in the custom alphabet. This allows O(1) comparison of characters.

3. Define a comparison function

Write a helper function that compares two strings lexicographically using the custom order. It should return true if the first string is less than or equal to the second.

4. Iterate and compare adjacent strings

Loop through the array from index 0 to n-2, comparing each pair of adjacent strings with the helper function. If any pair is out of order, return false immediately.

5. Return the result and analyze complexity

If all pairs are in order, return true. State the time complexity O(N * L) where N is the number of strings and L is the average length, and space complexity O(1) excluding the alphabet map.

Key Points to Mention

Mapping the custom alphabet to indices for efficient character comparison.
Handling the case where one string is a prefix of another (e.g., 'app' vs 'apple') — the shorter string comes first.
Early termination: return false as soon as an out-of-order pair is found.
Time complexity: O(N * L) where N is the number of strings and L is the maximum string length.
Space complexity: O(1) extra space if the alphabet size is fixed, or O(A) for the alphabet map.
Edge cases: empty array (return true), single string (return true), strings with characters not in the alphabet (if possible, handle appropriately).

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