Happy Number: which companies ask it and how to practise

Happy Number is a Easy problem that 24 companies are reported to ask. This page shows who, how often and how recently, what the question tests, and how to practise it out loud with an interviewer who asks follow-ups. We link to the original problem statement instead of copying it.

Which companies ask Happy Number?

Public company lists attribute Happy Number, a Easy problem, to 24 companies. The most frequent askers are BlackRock, Nike and Snowflake. 7 reports are from the last six months. It mainly tests hash table, math and two pointers. Use this as a practice guide, not a promise.

Practise this kind of question aloud

Who asks Happy Number

24 companies appear in the public lists for this question. By how often each is reported: BlackRock (very often), Nike (very often), Snowflake (very often), X (very often), Jump Trading (very often), Swiggy (very often), Accenture (often), Audible (often), Airbnb (often) and Microsoft (often), and 14 more. 7 of them were reported in the last six months, Snowflake, Audible and Microsoft among them.

These are candidate reports collected in public lists, not statements from the companies. They are a good guide to what to practise and a poor guide to what you will be asked: interviewers change questions, and the same company asks different questions by team and level. Last reviewed 25 October 2026.

What Happy Number tests

It is rated Easy, so interviewers expect a correct, complete solution and a clear statement of the time and space cost, often in the first 15 to 20 minutes, with time left for a variation.

A hash table trades memory for speed: remembering what you have seen turns a repeated search into a lookup. Interviewers want you to say that trade-off and the cost of collisions or a bad key.

Math questions reward finding the formula or the invariant before writing code, and watching for overflow, negatives and zero.

Two pointers walk a sorted or symmetric structure from both ends, or one fast and one slow. The proof that you never skip the answer is what the interviewer is waiting for.

How to practise it

Do not just solve it silently. In a real interview you explain the approach before you code, write while you talk, and defend the solution to follow-ups. Set a timer, say the brute force and its cost first, then improve it, and finish by stating the time and space complexity and two edge cases you tested.

On giveinterview you can take this kind of question as a spoken interview: the AI interviewer opens with the problem, watches you code, asks cross-questions and ends with a scorecard. Your first 3 single-question interviews are free for life, every interview report is free, and a new account also gets 6 welcome credits.

Read the original statement here: https://leetcode.com/problems/happy-number/ (opens the publisher's page). The statement is theirs; we do not reproduce it.

Questions to practise next

If you can solve Happy Number, try these next, chosen because they share its main topic and many of the same companies: Two Sum (Easy), LRU Cache (Medium), Longest Substring Without Repeating Characters (Medium), Group Anagrams (Medium), Top K Frequent Elements (Medium) and Subarray Sum Equals K (Medium). Each has its own page in the questions list.

Frequently asked questions

Is Happy Number asked at Amazon?

Amazon appears in the public lists for Happy Number, reported often. Treat that as a reason to practise it, not a promise you will be asked it.

How hard is Happy Number?