Alien Dictionary
A hard-tier problem at 37% community acceptance, tagged with Array, String, Depth-First Search. Reported in interviews at Pocket Gems and 12 others.
Alien Dictionary is a hard graph problem that appears in live assessments at Uber, Airbnb, X, and Citadel. You're given a sorted list of alien words and need to derive the order of characters in that alien alphabet. The catch: there's no obvious iteration pattern. Most candidates see the word list and freeze because the standard string-comparison intuition doesn't work. You need to recognize this as a topological sort problem hidden inside a dictionary. The acceptance rate sits at 37%, which means even strong engineers stumble here. If this problem surfaces in your live OA and you blank on the graph angle, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "Alien Dictionary"
Alien Dictionary is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe trick is building a directed graph where each edge represents a character ordering constraint, then performing topological sort. You compare adjacent words character by character: the first differing character tells you one character comes before another. The edge goes in the graph. Once you have all edges, use DFS or BFS to output the topological order. The gotcha: not every pair of words gives you information, and sometimes there are cycles (invalid input). Candidates often try greedy character comparison or sorting, which fails because you need to reason about global ordering constraints, not local ones. Array iteration and String operations set up the graph building, but the core insight is that this is a Graph problem requiring Topological Sort, typically via Depth-First Search or Breadth-First Search. If you haven't drilled topological sort patterns recently or haven't seen this exact disguise before, StealthCoder is the hedge for the live assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Alien Dictionary recycles across companies for a reason. It's hard-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Alien Dictionary interview FAQ
Is this actually asked at top companies or is it just in problem banks?+
Alien Dictionary has been reported at least 10 times across Uber, Airbnb, X, Citadel, Snap, Tencent, and others. It's a real live OA question, not a myth. The 37% acceptance rate confirms most people who encounter it fail the first time.
What's the actual trick if I blank on the approach?+
Recognize that comparing adjacent words in sorted order gives you character ordering rules. Each rule is an edge in a directed graph. Then topologically sort that graph using DFS or BFS. Bypass the instinct to compare all character pairs globally and instead extract constraints from word pairs only.
How does this relate to Topological Sort?+
Topological Sort is the core algorithm. Building the graph is the setup work. You need to know how to implement topo sort (DFS with a visited set and a result stack, or BFS with in-degree counts) to finish this problem. It's not just about spotting the pattern; you have to code it cleanly under pressure.
What's the difference between DFS and BFS solutions here?+
Both work. DFS uses a recursion stack and post-order traversal to build the result. BFS uses in-degree counts and a queue (Kahn's algorithm). DFS is slightly more intuitive for most people, but BFS is sometimes faster in practice. Pick whichever you code faster without bugs during the live assessment.
What are the edge cases that trip people up?+
Single character words, words where one is a prefix of another (no new ordering), cycles in the graph (invalid input), and empty character sets. Many candidates forget to check for prefix mismatches or don't validate the graph for cycles, causing wrong answers or crashes.
Want the actual problem statement? View "Alien Dictionary" on LeetCode →