Happy Number

Happy Number is a Easy Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Two Pointers pattern, in the Fast & Slow (Cycle Detection) subpattern.

Problem statement

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

Topics and companies

Topics: Hash Table, Math, Two Pointers.

Reported in interviews at Apple, Adobe, Barclays, ByteDance.

How to practise Happy Number on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(k), where k is the steps to be happy number time and O(k) space.

Open Happy Number in the code editor, or read the Happy Number editorial for a worked solution with its approach and complexity analysis.

Where Happy Number sits in the DSA pattern sheet

• Two Pointers pattern guide
• Fast & Slow (Cycle Detection) subpattern
• The full DSA patterns sheet
• All coding practice problems

Problems related to Happy Number

Other problems that use the same Fast & Slow (Cycle Detection) and Two Pointers techniques:

• Find the Duplicate Number — Medium, same subpattern
• Is Subsequence — Easy, same subpattern
• Linked List Cycle — Easy, same subpattern
• Backspace String Compare — Easy, same pattern
• Boats to Save People — Medium, same pattern
• Container With Most Water — Medium, same pattern