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

Solution：Hashset

思路： 看结果是否开始重复，或达到1
Time Complexity: .. Space Complexity: ..

Solution Code：

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

        }
        return false;
    }
}