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int fibWithRecursion(int n){
        // Of course the stack will explode before you can calc very large fibs.
	if (n < 2){
		return n;
	}
	return fibWithRecursion(n -1) + fibWithRecursion(n-2);
}

Dictionary<decimal,decimal> d = new Dictionary<decimal,decimal>();
decimal fib(decimal n){
        if (n<2) return n;
	// ### using memoization
	if (!d.ContainsKey(n)){
		decimal x = fib(n-1) + fib(n-2);
		d.Add(n,x);
	}
	return d[n];
}

decimal fibViaFor(decimal n){
	if (n == 0) return n;
	decimal prev = 0;
	decimal next = 1;
	
	for (int i = 1; i < n;i++){
		decimal oldPrev = prev;
		prev = next;
		next = oldPrev + next;
	}
	return next;
}

IEnumerable<decimal> fibViaIterator(decimal n){
	decimal prev =0;
	decimal next = 1;
	for (decimal i = 0; i < n; i++){
		yield return prev;
		decimal localFib = prev + next;
		prev = next;
		next = localFib;
	}
}

for (int x = 0; x<15;x++){
		Console.Write($"{fibWithRecursion(x)} ");
	}
	Console.Write("\n");
	
	for (int x = 0; x<15;x++){
		Console.Write($"{fib(x)} ");
	}
	Console.Write("\n");
	
	for (int x = 0; x<15;x++){
		Console.Write($"{fibViaFor(x)} ");
	}
	Console.Write("\n");
	
	foreach (decimal x in fibViaIterator(15)){
		Console.Write($"{x} ");
	}
	Console.Write("\n");

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377