Longest Word in Dictionary

Longest Word in Dictionary

What is this problem about?

The Longest Word in Dictionary coding problem gives you an array of strings representing a dictionary. Your objective is to find the longest string in the array that can be built one character at a time by other words present in the dictionary. If there is a tie for the longest word, you must return the one that comes first lexicographically (alphabetically).

Why is this asked in interviews?

This problem is highly favored by interviewers because it can be solved using two distinct approaches: a Hash Set with sorting, or a Trie (Prefix Tree). It evaluates a candidate's ability to understand prefix dependencies. The requirement to break ties using lexicographical order adds a layer of precision, testing whether you can carefully track multiple variables (length and string value) simultaneously.

Algorithmic pattern used

While sorting and a Hash Set work well (sort by length, check if word[:-1] is in the set, and build upwards), the Trie (Prefix Tree) is the more scalable, algorithmic pattern. You insert all words into a Trie. Then, you use Depth-First Search (DFS) or Breadth-First Search (BFS) to traverse the Trie, but you are only allowed to move to a child node if that node represents a complete, valid word from the dictionary (marked by an isEnd flag).

Example explanation

Dictionary: ["w", "wo", "wor", "worl", "world", "a", "ap"]

1. Using the Hash Set method: Sort the array lexicographically.
2. Initialize an empty string in the set "" so 1-letter words can build off it.
3. Check "a": is "" in set? Yes. Add "a" to set. Max = "a".
4. Check "ap": is "a" in set? Yes. Add "ap" to set. Max = "ap".
5. Check "w": is "" in set? Yes. Add "w".
6. Check "wo", "wor", "worl", "world"... all their prefixes are systematically found in the set. Max becomes "world" (length 5).

If we had "apple", but "app" was missing from the input, "apple" would be rejected because the chain was broken.

Common mistakes candidates make

A common mistake is failing to handle the lexicographical tie-breaker correctly. Candidates sometimes just return the first longest word they process, which might be incorrect depending on their sorting logic. If using a Trie, a frequent error is traversing down a path where an intermediate node is not a complete word (missing the isEnd flag), invalidating the "built one character at a time" rule.

Interview preparation tip

When tackling the Longest Word in Dictionary interview question, use the Hash Set method if you are short on time—it is very fast to implement. Sort the array first lexicographically, so that as you iterate, longer words automatically overwrite shorter ones, and ties are naturally resolved because the lexicographically smaller words were processed and stored first.