One of most common question from this area is to write a function which return Nth number of Fibonacci sequence.

Fibonacci numbers are a sequence of numbers defined by following formula:

ω

_{n}=ω

_{n-1}+ω

_{n-2}

The first number of the sequence is 0, the second number is 1, and each subsequent number is equal to the sum of the previous two numbers of the sequence itself, yielding the sequence 0, 1, 1, 2, 3, 5, 8, etc. (Wikipedia).

Being very simple from the one side and quite sophisticated from the other Fibonacci numbers are the perfect candidate for the technical coding interview. During my last year job hunt, I actually end up answering questions about Fibonacci numbers on every other interview I had.

** **

**Problem 1.** *Write a function which return Nth number of Fibonacci sequence *

The most simple and the most popular solution you would see during the interview employ recursive nature of the sequence.

Possible solution #1 – Recursion

```
```

static int Fibonacci (int n)

{

if(n < 0)

throw new ArgumentException(\"Input parameter invalid\");

int N;

if (n == 0) N = 0;

else

if (n == 1 || n == 2) N = 1;

else

N = Fibonacci(n - 1) + Fibonacci(n - 2);

return N;

}

While totally valid, this is probably the worst solution you can give during the interview. Recursion in this case is total overkill as it requires keeping all sequence in the memory.

__Possible solution #2 – Iteration __

```
```

static int Fibonacci (int n)

{

int index1 = 1, index2 = 1;

if (n == 0) return 0;

if (n == 1 || n == 2) return 1;

int current;

for (int i = 2;i<= n;i++)

{

current = index1+index2;

index1=index2;

index2= current;

}

return current;

}

__Possible solution #3 – Computational (Closed form expression)__

This is simple calculation of closed form expression for Fibonacci numbers

```
```

static double Fibonachi(int n)

{

double gr = ((double)1 + Math.Sqrt(5)) / 2;

double grN = Math.Pow(gr, n);

double gr_1 = Math.Pow(((double)1 - gr), n);

return (grN-gr_1)/Math.Sqrt(5);

}

**Problem 2.** *Write a function which would print Fibonacci numbers up to given maximum*

While, all solutions above are very easy to modify to fit this scenario it is good if you can through something extra in your answer to stand out of the crowd. So in order to do so, I would use enumerator. Certainly be prepare to discuss how yield operator works and what might resulting code look like then compiled into MSIL.

```
```

static IEnumerable Fibonacci(double maxPosition)

{

double current = 1;

double previous = 0;

for (double i = 0;i<maxPosition;i++)

{

if (i > 1)

{

var oldCurrent = current;

current = current + previous;

previous = oldCurrent;

yield return current;

}

else

{

yield return i == 0 ? previous : current;

}}}

Problem 3.

*Write a function which would check if a given number belongs to Fibonacci sequence*

The easiest way is to generate Fibonacci numbers till this number and see if this number is one of them and again it probably the answer interviewer is looking for. However there is couple over ways which might impress interviewer.

__Possible solution:__

It turns out that a positive integer ω is a Fibonacci number if and only if one of 5ω^{2} + 4 and 5ω^{2} - 4 is a perfect square [The (Fabulous) FIBONACCI Numbers by Alfred Posamentier and Ingmar Lehmann]

```
```

bool isFibonacci(int w)

{

double X1 = 5 * Math.Pow(w, 2) + 4;

double X2 = 5 * Math.Pow(w, 2) - 4;

long X1_sqrt = (long)Math.Sqrt(X1);

long X2_sqrt = (long)Math.Sqrt(X2);

return (X1_sqrt*X1_sqrt == X1) || (X2_sqrt*X2_sqrt == X2) ? true : false;

}

As I always iterate, try to direct interview into area you know well. As I already mentioned, one approach for this particular question is to provide iterator solution and demonstarte your advance knowledge. Another is to talk about strategy pattern as this is a really good demonstration for different strategies you might want to use in different curcumstances. If you want to learn more Mathworld provides very nice overview:http://mathworld.wolfram.com/FibonacciNumber.html