Nth fibonacci

nthFibonacci

using System;

namespace NthFibonacci
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int n = 4;
            Console.WriteLine("{0}th fibonacci is {1}, Expected {2}", n, nthFibonacci(n), 3);
        }
        
        // O(n) time
        // O(1) space
        static int nthFibonacci(int n) {
            if (n < 0) {
                throw new Exception("Invalid input");
            }
            
            if (n == 0 || n == 1) {
                return n;
            }
            
            int result = 0;
            
            int result1 = 0;
            int result2 = 1;
            for(int i = 2; i <= n; i++) {
                result = result1 + result2;
                result1 = result2;
                result2 = result;
            }
            
            return result; 
        }
    }
}