Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

Meta SWE interview with a graph-based coding question. Pretty standard algorithmic stuff but the follow-up had some real teeth to it.

Questions Asked (1)

Given a sorted array of strings in an unknown alien alphabet, determine a valid character ordering for that alphabet. Return an empty string if no valid ordering exists (e.g. due to a cycle).

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The core insight is building a directed graph from adjacent word comparisons and running topological sort.

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AI HintsAI Generated

Suggested Approach

Model the problem as a directed graph where each character is a node and edges represent the relative order inferred from adjacent words. Then perform a topological sort to find a valid ordering, returning an empty string if a cycle is detected.

Pro tip: Clarify edge cases upfront, such as invalid input (e.g., a longer word before a shorter one with the same prefix) and the possibility of multiple valid orderings. Mention that any valid topological order is acceptable, and discuss how to handle disconnected components.

1. Build the Graph

Iterate through adjacent word pairs and compare characters to find the first differing character. Add a directed edge from the first character to the second. If no differing character is found and the first word is longer, return an empty string as the input is invalid.

2. Detect Cycles and Topological Sort

Perform a topological sort using either Kahn's algorithm (BFS with in-degrees) or DFS with cycle detection. If a cycle is detected, return an empty string.

3. Handle Disconnected Components

After processing all edges, some characters may not appear in any edge. Include them in the ordering arbitrarily, as their relative order is unconstrained.

4. Return the Ordering

If no cycle is detected, return the topological ordering as a string. If a cycle exists, return an empty string.

Key Points to Mention

Graph representation: adjacency list and in-degree array for efficient processing.
Cycle detection: using Kahn's algorithm (if processed nodes < total nodes, cycle exists) or DFS with recursion stack.
Time and space complexity: O(C) where C is the total number of characters in all words, and O(1) or O(U) space where U is the number of unique characters.
Edge cases: empty input, single word, words with identical prefixes but different lengths (invalid if longer word comes first).
Multiple valid orderings: any valid topological order is acceptable; mention that the problem may not have a unique solution.
Handling characters not present in any edge: they can be placed anywhere in the ordering.

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