Haskell中的将军树=玫瑰树？ 一棵玫瑰树的驿站吗？

Question

I was supposed to create a data structure for a Tree, where every Node has an undefined amount of branches. I am guessing this will be a rose tree.

data GTree a = Node a [GTree a] 

Now I am supposed to write a postorderG function that will give me a list of all my elements in my general in a postorder sequence I wrote this but it does not seem right... Could someone help me?

postorderG :: GTree a -> [a]
postorderG (Node x l r) = postorder l ++ postorder r ++ [GTree x]

Answer 1

GTree is a type constructor, not a data constructor; a tree would be created with Node x [] , not GTree x .

However , you don't need to create a tree at all here. You just need a the value stored at the root of the input tree for the final list in the return value.

postorderG :: GTree a -> [a]
postorderG (Node x ch) =  concatMap postorderG ch -- [a]
                          ++ [x]         -- [a], not [GTree a]

If you wanted to, you could create a singleton tree to append to ch , the apply postOrderG to each child and the new singleton in order.

postorderG (Node x ch) = concatMap postorderG (ch ++ [Node x []])

Taking advantage of the [] monad instead of using concatMap , the RHS would look like either

(ch >>= postorderG) ++ [x]

or

(ch ++ [Node x []]) >>= postorderG