小编Jay*_*yyy的帖子

  type BSTree a = BinaryTree a

  data BinaryTree a = Null | Node (BinaryTree a) a (BinaryTree a)
                      deriving Show

  flattenTree :: BinaryTree a -> [a]
  flattenTree  tree = case tree of
      Null -> []
      Node left val right -> (flattenTree left) ++ [val] ++ (flattenTree right)

  isBSTree :: (Ord a) => BinaryTree a -> Bool
  isBSTree btree = case btree of
      Null -> False
      tree -> (flattenTree tree) == sort (flattenTree tree)

我想做的是编写一个函数来确定给定的树是否是二叉搜索树，我的方法是将所有值分组到一个列表中并导入Data.List，然后对列表进行排序以查找它们是否相等，但是有点复杂。我们可以不导入其他模块就能做到吗？

  data BinaryTree a = Null | Node (BinaryTree a) a (BinaryTree a)
             deriving Show
  data RoseTree a = RoseNode a [RoseTree a]
     deriving Show
  binaryTreeToRose :: BinaryTree a -> RoseTree a
         binaryTreeToRose btree = case btree of
         Node Null a Null -> RoseNode a []
         Node left a Null -> RoseNode a [binaryTreeToRose left]
         Node Null a right -> RoseNode a [binaryTreeToRose right]
         Node left a right -> RoseNode a [binaryTreeToRose left]++[binaryTreeToRose right]

I try to write a function to …