Memoizing Fibonacci Numbers

using System;
class Program
{
    /// <summary>
    /// Finds the nth Fibonacci number.
    /// </summary>
    /// <param name="num">An integer number.</param>
    /// <param name="lookupTable">Stores already calculated Fibonacci numbers.</param>
    /// <returns>The nth Fibonacci number.</returns>
    public static int Fib(int num, int[] lookupTable)
    {
        if (lookupTable[num] == -1) // Check if already present
        {
            // Adding entry to table when not present
            if (num <= 1)
            {
                lookupTable[num] = num;
            }
            else
            {
                lookupTable[num] = Fib(num - 1, lookupTable) + Fib(num - 2, lookupTable);
            }
        }
        return lookupTable[num];
    }
    // Driver code to test above method
    public static void Main(string[] args)
    {
        int num = 6; // Finding the nth Fibonacci number 
        int[] lookupTable = new int[num + 1]; // Initializing the lookup table to have -1 of size num + 1
        for (int i = 0; i <= num; i++)
        {
            lookupTable[i] = -1;
        }
        Console.WriteLine(Fib(num, lookupTable)); // Output: 8
    }
}