Prolog - Count the number of leaves in a tree

If I have a tree that for example looks like this:

tree3(b(l(1),b(l(2),l(3)))).

How would I write a program that counts the number of leaves? I want it to look something like this when it's been used:

?- tree3(T), count_lea开发者_如何转开发ves(T, N).

N = 3,
T = b(l(1),b(l(2),l(3)))

I'd love any help!

You could do it like this:

count_leaves(l(_), 1).
count_leaves(b(B1, B2), N) :- count_leaves(B1, N1), count_leaves(B2, N2), N is N1 + N2.

Basically, tree that is just a leaf has one leaf. If the tree starts with a branch, recurse into both branches and add the results.

Your solution gives you no, because nothing will match against empty. And even if you fixed that, you wouldn't be counting leaves, but inner nodes.