Longest Harmonious Subsequence
A easy-tier problem at 57% community acceptance, tagged with Array, Hash Table, Sliding Window. Reported in interviews at LiveRamp and 1 others.
Longest Harmonious Subsequence sits at the intersection of hash tables and array counting, and it's easier than you think until you misread the problem. The trick is in what "harmonious" actually means: a subsequence where the difference between max and min is exactly 1. You've seen candidates crush harder problems and blank on this one because they're overthinking it or trying to sort when they shouldn't. It shows up in assessments at LiveRamp and ZS Associates. If you hit this during a live OA and the definition catches you off-guard, StealthCoder solves it in seconds flat, invisible to the proctor.
Companies that ask "Longest Harmonious Subsequence"
Longest Harmonious Subsequence 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. Made by an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too.
Get StealthCoderThe naive approach is to sort or use a sliding window, but this problem doesn't care about order. What matters is that you count occurrences of each number, then find pairs of numbers that differ by exactly 1. Hash table is the right tool: scan the array once, count frequencies, then iterate through the counts and sum up pairs (num and num+1). Most candidates lose marks by overcomplicating it with two-pointer logic or sliding windows that add no value. The acceptance rate hovers around 57 percent, which tells you some people are nailing it cleanly while others are stuck on implementation details. When you hit this live and blank on whether to sort first, StealthCoder runs invisibly during screen share and shows you the hash table solution in real time.
Pattern tags
You know the problem.
Make sure you actually pass it.
Longest Harmonious Subsequence recycles across companies for a reason. It's easy-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. Made by an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Longest Harmonious Subsequence interview FAQ
What does 'harmonious' mean in this problem?+
A harmonious subsequence has a max and min value that differ by exactly 1. You're not looking for consecutive elements in the array, just numbers whose values are consecutive. For example, [1, 1, 1, 2, 2] is harmonious (max 2, min 1, diff 1) but [1, 1, 3, 3] is not (max 3, min 1, diff 2).
Should I sort the array first?+
No. Sorting wastes time and adds complexity. Use a hash table to count frequencies in one pass, then scan the counts to find pairs where num and num+1 both exist. That's it. Sorting is a red herring on this problem.
Is this actually asked at FAANG or tier-one companies?+
It shows up at LiveRamp and ZS Associates in their online assessments. It's not a common ask at the biggest names, but it's a frequent warm-up or medium-tier problem in company-specific pipelines. Acceptance rate is 57 percent, so most people who see it pass.
What's the time complexity and why does it matter?+
Hash table approach is O(n) time and O(n) space. You iterate the array once to count, then iterate the keys once to find harmonious pairs. That's optimal. Sorting would be O(n log n), which fails an efficiency check if the interviewer is strict.
How does this relate to sliding window problems?+
It doesn't, really. Sliding window requires order to matter. Here, order is irrelevant. You're just counting and pairing numbers by value. If you reach for sliding window, you're solving a harder problem than the one being asked. Hash table is the right pattern.
Want the actual problem statement? View "Longest Harmonious Subsequence" on LeetCode →