using System;
public class Program
{
public static void Main()
//0,1,1,2,3,5,8,13,21...
/*
Fib(0); // => 0
Fib(1); // => 1
Fib(2); // => 1
Fib(3); // => 2
Fib(4); // => 3
...
Fib(7); // 13
*/
Console.WriteLine(Fib(7));
}
public static int Fib(int n)
// Edge cases:
if (n < 0)
throw new ArgumentException("Index was negative. No such thing as a negative index in a series.");
if (n == 0 || n == 1)
return n;
// We'll be building the fibonacci series from the bottom up.
// So we'll need to track the previous 2 numbers at each step.
int prevPrev = 0; // 0th fibonacci
int prev = 1; // 1st fibonacci
int current = 0; // Declare and initialize current
for (int i = 1; i < n; i++)
// Iteration 1: current = 2nd fibonacci
// Iteration 2: current = 3rd fibonacci
// Iteration 3: current = 4th fibonacci
// To get nth fibonacci ... do n-1 iterations.
current = prev + prevPrev;
prevPrev = prev;
prev = current;
return current;
