haskell - 在 haskell 中将树缩减为列表

Question

我正在尝试将树减少到列表，但我被困在这里..任何建议表示赞赏..

data Tree a = Leaf a | Node (Tree a) a (Tree a) deriving (Eq, Show)
treeToList :: (Ord a) => Tree a -> [a]
treeToList (Node root left right) = treeToList left ++ [root] ++   treeToList right

预期结果:

ghci> treeToList (Node (Leaf 1) 2 (Node (Leaf 3) 4 (Leaf 5)))

[1,2,3,4,5]

Answer 1

您有两个错误:

1. 非详尽模式(针对 Leaf 构造函数)。
2. 在 Node 模式中，您将 root 匹配为左树值 (Tree a)，而不是 a。

因此结果应如下所示:

data Tree a = Leaf a | Node (Tree a) a (Tree a) deriving (Eq, Show)

treeToList :: (Ord a) => Tree a -> [a]
treeToList (Leaf v) = [v]
treeToList (Node left root right) = treeToList left ++ [root] ++ treeToList right

*Main> treeToList (Node (Leaf 1) 2 (Node (Leaf 3) 4 (Leaf 5)))
[1,2,3,4,5]