Find nth fibonacci number.

The Fibonacci Sequence is the series of numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, …

The next number is found by adding up the two numbers before it.

Let F[i] be the ith fibonacci number

F[0]=0
F[1]=1
F[i]=F[i-1]+F[i-2]

Solution using Recursion

using System;
namespace Fibonaccinumber{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("Enter the Number");
            var n = Convert.ToInt32( Console.ReadLine());
            var result = Fibonacci(n);
            Console.WriteLine($"Fibonacci number is: {result}");
            Console.ReadLine();
        }
        
            static int Fibonacci(int n)
            {
                if (n <= 1)
                return n;
                return Fibonacci(n - 1) + Fibonacci(n - 2);
            }
    }
    
}

Solution Dynamic Dynamic Programming

using System; namespace Fibonaccinumber{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("Enter the Number");
            var n = Convert.ToInt32(Console.ReadLine());
            var result = Fibonacci(n);
            Console.WriteLine($"Fibonacci number is: {result}");
            Console.ReadLine();
        }
            
        static int Fibonacci(int n)
        {
            var arr = new int[n + 1];
            arr[0] = 0;
            arr[1] = 1;
            for (var i = 2; i <= n; i++)
            arr[i] = arr[i - 1] + arr[i - 2];
            return arr[n];
        }
    }
    
}