Citadel·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Citadel software engineer interview with a graph/topology problem. Pretty standard for this kind of role but the problem has enough edge cases to trip you up if you're not careful.

Questions Asked (1)

Given a sorted list of words from an alien language that uses the English alphabet, derive the ordering of letters in that language. Return the letters sorted by the alien language's rules, an empty string if no valid ordering exists, or any valid ordering if multiple exist.

Algorithms & Data Structures

Author's notes

Classic topological sort problem once you see it, but getting there takes a minute.

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AI HintsAI Generated

Suggested Approach

Model the problem as a directed graph where each letter is a node and edges represent the relative order inferred from adjacent words. Perform a topological sort to produce a valid letter ordering, detecting cycles to return an empty string if no valid ordering exists.

Pro tip: Explicitly handle edge cases like duplicate words, prefix relationships (e.g., 'abc' before 'ab'), and disconnected graphs; these are common pitfalls that interviewers at Citadel look for.

1. Build the graph

Initialize a graph with all unique letters as nodes. For each pair of adjacent words, compare characters to find the first differing position and add a directed edge from the first word's character to the second's. If the first word is longer and is a prefix of the second, the ordering is invalid.

2. Compute in-degrees

Calculate the in-degree for each node by counting incoming edges. This will be used to identify starting points for topological sorting.

3. Topological sort

Use Kahn's algorithm (BFS with a queue) or DFS to generate a topological ordering. Start with nodes having in-degree zero, and repeatedly remove them while updating in-degrees of neighbors.

4. Detect cycles and validate

If the topological sort does not include all nodes, a cycle exists, meaning no valid ordering. Return an empty string. Otherwise, return the ordering.

5. Handle multiple valid orderings

If multiple valid orderings exist, any one is acceptable. The algorithm naturally produces one valid ordering; no special handling is needed.

Key Points to Mention

Graph representation: adjacency list and in-degree array.
Topological sorting algorithms: Kahn's (BFS) vs. DFS-based.
Cycle detection: if the sorted order length is less than the number of unique letters, return empty string.
Edge cases: duplicate words, prefix relationships, and disconnected graphs.
Time and space complexity: O(N * L) where N is number of words and L is max word length, space O(1) for fixed alphabet size.
Handling of multiple valid orderings: any valid topological order is acceptable.

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