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

當某個數不是happy number的時候如何退出迴圈：

用set記錄每個數字的 sum of every digit square，一旦當前數字的sum不能被加入set（說明之前出現過這個數，此後會一直進行相同的迴圈），返回false

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