Задача: Часто встречаемый элемент *

FindFreqNumber.cs

using System;
					
public class Program
{
	public static int FindFreqNumber(int[] arr) 
	{
		int len = arr.Length;
		if (len < 3)
		{
		    return arr[0];
		}

		int size = len / 4;
		for (int i = 0; i < len - size; i++)
		{
		    if (arr[i] == arr[i + size])
		    {
			return arr[i];
		    }
		}

		return -1;
	}
	
	public static void Main()
	{
		Console.WriteLine("UniLecs");
		
		// tests
		Console.WriteLine(FindFreqNumber(new int[] { 1, 3, 3, 7, 7, 7, 7, 8, 10 })); // 7
		Console.WriteLine(FindFreqNumber(new int[] { 1, 2 })); // 1 или 2
	}
}