2008/4/28 cetin tozkoparan :

Assume a tree is a subtree of the other if all elements of the first tree is included in the second with the exact structure; all parent-child relations are preserved with their order.

data Tree = Empty | Leaf Int | Node (Int,Tree,Tree)

Bit of a nitpick: Haskell folks would usually leave off the tuple on the Node constructor, since constructors tuple for you already. So Tree would become: data Tree = Empty | Leaf Int | Node Int Tree Tree But it's (almost) equivalent either way.

subtree:: Tree -> Tree -> Bool

How can i start to write this code with this data structure?

The way most tree algorithms are done is by recursion. Maybe it would help if you saw the problem in a more explicit form: A tree t is a subtree of a tree u if: t = u, u is a Node and t is a subtree of the left child, or u is a Node and t is a subtree of the right child. That definition should roughly translate into an inefficient implementation. There's probably a more efficient one somewhere, but it's somewhere other than the top of my head. Luke