202. 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: 

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

Analysis:
Use hash set to cache the sum, if sum exists in set, then the algorithm falls into cycle. So its not happy number.
Solution:

class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set  = new HashSet<>();
        int sum = 0;
        while (set.add(sum)) {
            sum = 0;
            while (n > 0) {
                sum += (n % 10) * (n % 10);
                n = n / 10;
            }
            if (sum == 1) return true;
            n = sum;
        }
        return false;
    }
}