DSA interview question

Java Collections: Top K Frequent Words

Easy · Algorithms · Topics: collections, java

The problem

The same leaderboard as in the C++ STL exercise, but your team's backend is in Java: count the words users search for and return the k most frequent, with ties broken alphabetically. The task doubles as a tour of the Java Collections Framework that interviews in Java assume you know: HashMap for counting, ArrayList and List.sort with a Comparator for ranking, plus TreeSet, PriorityQueue, ArrayDeque and TreeMap.

Example: words ["apple", "banana", "apple", "cherry", "banana", "apple"], k = 2 → [apple=3, banana=2].

Function signature

static List<Map.Entry<String, Integer>> topKFrequent(List<String> words, int k);
// sorted by count descending, then word ascending; at most k entries

Test cases

Input `(words, k)`	Output	Explanation
`(["apple","banana","apple","cherry","banana","apple"], 2)`	`[apple=3, banana=2]`	Highest counts first
`(["b","a","c","a","b","c"], 3)`	`[a=2, b=2, c=2]`	All tied, alphabetical
`(["x"], 5)`	`[x=1]`	Fewer distinct words than k
`(["dog","cat","dog"], 1)`	`[dog=2]`	Only the top entry

(Confirm: ties are broken alphabetically; return every word if there are fewer than k.)

How interviewers phrase it

"Top k frequent words" (LeetCode 692) in a Java interview.
Collections theory: "HashMap vs TreeMap?", "ArrayList vs LinkedList?", "How does HashMap work internally?", "Why override hashCode when you override equals?"

Clarifying questions to ask

Tie-breaking order?
Fewer than k distinct words: return all?
Case-sensitive?
Is the input large enough that a heap would beat a full sort?

Follow-ups to expect

"HashMap vs TreeMap?"
"How does HashMap work?"
"Why must equals and hashCode agree?"
"ArrayList vs LinkedList?"
"How do you make a max-heap?"
"Why Integer.compare instead of subtracting in a comparator?"

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