Consider the following function $fn.$

int $fn(n)\{$

if $(n==0))$

return $1$ ;

$\}$

if $(n==1)\{$

return $2$ ;

$\}$

return $2**fn(n-2)+fn(n-1)$;

$\}$

$($a$)$ Compute the value $fn(5).$ Show the steps in your computation. draw the recursive tree What does fn$($n$)$ compute for n $>=0?$ Justify your answer.

$($d$)$ Compute an estimate of the execution time as a function of n$.$ Give

your answer in Big$-$Oh notation and justify your answer. answer by handwritting please