Happy Number

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

Naive Way: The statement is describing a recursive process. So it is obvious that the question want me to write a recursive method. The logic has been stated and clear. Base case (ending condition) is either n=1 or n has been visited. And since I need to mark whether a number has been visited, a hashset is required.

 public class Solution {  
   Set<Integer> visited = new HashSet<Integer>();  
   public boolean isHappy(int n) {  
     // base case  
     if(n==1) return true;  
     // visited  
     if(visited.contains(n)) return false;  
     // main processing  
     int sum = 0;  
     visited.add(n);  
     while(n!=0){  
       int lastDigit = n % 10;  
       sum += lastDigit * lastDigit;  
       n /= 10;  
     }  
     return isHappy(sum);  
   }  
 }