Is Happy Number easy or hard?

Happy Number is classified as an Easy problem. The main challenge is recognizing that the process forms a cycle and applying either a HashSet or Floyd's cycle detection to stop infinite loops.

Happy Number Python/Java solution

In Python or Java, repeatedly compute the sum of the squares of digits and track visited values using a set. Continue until the number becomes 1 or a previously seen value appears. Both implementations run in O(log n) time with simple loops and digit extraction.

How to solve Happy Number in O(1) space?

Use Floyd's cycle-finding algorithm with slow and fast pointers. The slow pointer applies the digit-square transformation once per step, while the fast pointer applies it twice. If the pointers meet at a number other than 1, the sequence forms a cycle and the number is not happy. This method runs in O(log n) time and O(1) space.

What is the best approach for Happy Number?

The most common approach uses a HashSet to detect cycles while repeatedly computing the sum of the squares of digits. If the sequence reaches 1, the number is happy. If a value repeats, a cycle exists and the number is not happy. This approach runs in O(log n) time per sequence step and uses O(log n) space.

Is Happy Number asked at Google/Amazon/Meta?

Happy Number is a common easy-level screening question and has appeared in interviews at companies like Amazon and Microsoft. It tests understanding of cycle detection, hashing, and number manipulation rather than complex algorithms.

What data structure is used in Happy Number?

The typical solution uses a HashSet to store previously seen numbers while generating the sequence. The set allows constant-time lookup to detect when the sequence repeats and enters a cycle.

What is the time complexity of Happy Number?

The time complexity is O(log n) because each iteration processes the digits of the number, and the number of digits grows logarithmically with n. The sequence length is small in practice since values quickly shrink into a bounded cycle. Space complexity is O(log n) with a HashSet or O(1) with Floyd's cycle detection.

Description Solution Video 10:59

202. Happy Number

Easy60.0% acceptance27 companies

Write an algorithm to determine if a number n is happy.

A happy number is a number defined by the following process:

Starting with any positive integer, replace the number by the sum of the squares of its digits.
Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
Those numbers for which this process ends in 1 are happy.

Return true if n is a happy number, and false if not.

Example 1:

Input: n = 19
Output: true
Explanation:
1² + 9² = 82
8² + 2² = 68
6² + 8² = 100
1² + 0² + 0² = 1

Example 2:

Input: n = 2
Output: false

Constraints:

1 <= n <= 2³¹ - 1

Asked at top 20 of 27 companies

Snowflake Twitter BlackRock Nike Audible Jump Trading Swiggy Accenture Airbnb Visa Tesla

Google

+8 more

Amazon Microsoft Bloomberg Uber

Cisco JPMorgan Chase Agoda Meta

Topics

10:59

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution

202. Happy Number

Asked at top 20 of 27 companies

Topics

Similar problems