DSA interview question

Highest and Lowest Occurring Elements in an Array

Easy · Algorithms · Topics: hashing, array

The problem

A music-streaming dashboard wants two numbers from today's play log (an array of song IDs): the most-played song and the least-played song among those that were played at all. When several songs tie, the dashboard shows the one with the smallest ID so the answer is deterministic. It's a direct follow-on from counting frequencies: once you have a frequency table, both answers come from a single pass over it, as long as you get the tie-breaking right.

Example: [10, 5, 10, 15, 10, 5] → most frequent 10 (3 times), least frequent 15 (once).

Function signature

pair<int, int> highestAndLowestFrequency(const vector<int>& arr);  // {most frequent, least frequent}; arr non-empty

Test cases

Input `arr`	Output	Explanation
`[10, 5, 10, 15, 10, 5]`	`{10, 15}`	10 appears 3 times, 15 once
`[2, 2, 3, 4, 4, 2]`	`{2, 3}`	2 appears 3 times; 3 once, 4 twice
`[1, 2, 3]`	`{1, 1}`	All tie at 1, so the smallest value wins both
`[7, 7, 1, 1]`	`{1, 1}`	Tie at 2 for both; smallest value 1
`[42]`	`{42, 42}`	Single element

(Confirm: the array is non-empty; ties are broken by the smallest value; "least frequent" only considers values that appear.)

How interviewers phrase it

"Find the element that occurs most often in an array."
"Find the highest- and lowest-frequency elements" (Striver's hashing lecture).
Follow-on problems: majority element (> N/2 times), top-k frequent elements.

Clarifying questions to ask

How should ties be broken: smallest value, first occurrence, or any?
Does "least frequent" mean among values present in the array?
What's the value range? (Array vs hash map.)
Is the array guaranteed non-empty?

Follow-ups to expect

"Can you do it in one pass over the array?"
"What if the element must appear more than N/2 times?"
"How would you return the k most frequent?"
"Complexity?"

Practise questions like this with a real engineer

On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the expected answer to every follow-up, hints to give if you stall, and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.

Book a free mock interview