[Leetcode] Happy Number

Problem

Write an algorithm to determine if a number 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, and 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 numbers.

Example: 19 is a happy number

• 12 + 92 = 82
• 82 + 22 = 68
• 62 + 82 = 100
• 12 + 02 + 02 = 1

Analysis

• Since the computation is a recursive process, so that we can recursive check whether a number is a happy number
• We need to maintain a hashset to track whether a number has shown up before
• If yes, then it will not be possible to reach 1 in the future

Code