What does this recursive method do$?$

private int unknown$($Node n$)$

$\{$

if $($n $==$ null$)$

return $0$;

else if $($n$.$left $==$ null && n$.$right $==$ null$)$

return $1$ ;

else if$($n$.$left $!=$ null && n$.$right $!=$ null$)$

return $(1+$ unknown$($n$.$left$)+$ unknown$($n$.$right$))$;

else

return $($unknown$($n$.$left$)+$ unknown$($n$.$right$))$;

$\}$

count the number of leaves nodes

count the number of leaf nodes and nodes that have two children.

count the number of nodes that have two children in the binary tree and one child

count number of non$-$leave nodes