Daily Challenge #JS-105: Check if a Number is a Happy Number

JS-105.cs

/*
A happy number is a number which eventually reaches 1 
when replaced repeatedly by the sum of the square of 
its digits. If it loops endlessly in a cycle, 
then it is not a happy number.
For example, starting with 19, the sequence is: 1² + 9² = 82, 
                            8² + 2² = 68, 
                            6² + 8² = 100, 
                            1² + 0² + 0² = 1. 
Hence, 19 is a happy number. 

Implement a function to check if a given number is a happy number.
https://en.wikipedia.org/wiki/Happy_number
*/


void Main()
{
	IsHappyNumber(13).Dump();
	IsHappyNumber(4).Dump();
	IsHappyNumber(320).Dump();
	IsHappyNumber(321).Dump();
}

HashSet<int> HappyNums = new HashSet<int>();
HashSet<int> NonHappyNums = new HashSet<int>();

bool IsHappyNumber(int num)
{
	var foundNums = new HashSet<int>();
	while (num != 1)
	{
		if (HappyNums.Contains(num))
			break;
		
		if(NonHappyNums.Contains(num)|| foundNums.Contains(num))
		{
			AddRange(NonHappyNums, foundNums);
			return false;
		}
		foundNums.Add(num);
		num = SumDigitsSquared(num);
	}

	foundNums.Add(num);
	AddRange(HappyNums, foundNums);
	return true;
}

int SumDigitsSquared(int num) => Digits(num).Select(n => n * n).Sum();


IEnumerable<int> Digits(int n)
{
	while (n > 0)
	{
		yield return n % 10;
		n = n /10;
	}
}

void AddRange(HashSet<int> hs, IEnumerable<int> others)
{
	foreach (int n in others)
		hs.Add(n);
}