Solution to Missing Numbers problem on HackerRank

MissingNumberSolution.cs

void Main()
{
	var size_a = int.Parse(Console.ReadLine());
	var list_a = Console.ReadLine()
		.Split(' ')
		.Take(size_a)
		.Select(x => int.Parse(x))
		.ToList();
		
	var size_b = int.Parse(Console.ReadLine());
	var list_b = Console.ReadLine()
		.Split(' ')
		.Take(size_b)
		.Select(x => int.Parse(x))
		.ToList();
	
	var hashMapA = new Dictionary<int, int>();
	var hashMapB = new Dictionary<int, int>();

	foreach (var i in list_a)
	{
		hashMapA.InsertOrUpdate(i);
	}

	foreach (var i in list_b)
	{
		hashMapB.InsertOrUpdate(i);
	}

	var missingValues = new HashSet<int>();

	foreach (var key_a in hashMapA.Keys)
	{
		if (!hashMapB.ContainsKey(key_a))
		{
			missingValues.Add(key_a);
		}
		else
		{
			var count_a = hashMapA[key_a];
			var count_b = hashMapB[key_a];

			if (count_a - count_b != 0)
			{
				missingValues.Add(key_a);
			}
		}
	}

	foreach (var key_b in hashMapB.Keys)
	{
		if (!hashMapA.ContainsKey(key_b))
		{
			missingValues.Add(key_b);
		}
		else
		{
			var count_a = hashMapA[key_b];
			var count_b = hashMapB[key_b];

			if (count_a - count_b != 0)
			{
				missingValues.Add(key_b);
			}			
		}
	}
	
	Console.WriteLine(missingValues.OrderBy(_ => _).Aggregate(string.Empty, (acc, x) => acc + " " + x).TrimStart());
}

public static class DictionaryExtension
{
	public static void InsertOrUpdate(this Dictionary<int, int> hashMap, int key)
	{
		int val;

		if (hashMap.ContainsKey(key))
		{
			hashMap[key]++;
		}
		else
		{
			hashMap.Add(key, 1);
		}
	}
}