haskell - 从树中删除一个节点并返回结果森林

Question

我有 Java 背景，想学习一些 Haskell。不过目前有点卡住了。

我想要做的是:我有一个树列表，其中每个节点在列表中的所有树中都有唯一的标识符。现在我想从这些树之一中删除一个节点并返回新树以及未更改的树。

删除节点应该:

• 使该节点的所有子节点成为新树的根
• 删除已删除节点的父节点，直到根节点(包括根节点)，并对所有已删除节点执行上述操作

想象一下以下树:

当我删除节点“2”时，我希望结果是以下树:

树中的每个节点都由标识符和子树列表组成。 这是我到目前为止所拥有的，但它显然不起作用，而且我对如何用 Haskell 解决这个问题有点迷失:

import Data.Tree
data CustomNode = CustomNode { identifier :: Int } deriving (Ord,Eq,Show,Read)
type CustomTree = Tree CustomNode

myTree0 = t0
    where
        leaf i = Node CustomNode{identifier = i} []
        t0 = Node CustomNode{identifier = 0} [t1]
        t1 = Node CustomNode{identifier = 1} [t2, t5]
        t2 = Node CustomNode{identifier = 2} [leaf 3, leaf 4] 
        t5 = Node CustomNode{identifier = 5} [leaf 6]

myTree1 = t0
    where
        leaf i = Node CustomNode{identifier = i} []
        t0 = Node CustomNode{identifier = 7} [leaf 8]

deleteNode :: Int -> [CustomTree] -> [CustomTree]
deleteNode _ [] = []
deleteNode n (x:xs) = if isNodeInTree n x then deleteNodeFromTree n x ++ xs else deleteNode n xs
--below is the fixed line as per the answer below
--deleteNode n (x:xs) = if isNodeInTree n x then deleteNodeFromTree n x ++ xs else x : deleteNode n xs

deleteNodeFromTree :: Int -> CustomTree -> [CustomTree]
deleteNodeFromTree n (Node c xs) = if identifier c == n then [] else deleteNode n xs
--below is the fixed line as per the answer below
--deleteNodeFromTree n (Node c xs) = if identifier c == n then xs else deleteNode n xs

isNodeInTree :: Int -> CustomTree -> Bool
isNodeInTree n (Node c xs) = if identifier c == n then True else isNodeInForest n xs

isNodeInForest :: Int -> [CustomTree] -> Bool
isNodeInForest n [] = False
isNodeInForest n (x:xs) = if isNodeInTree n x then True else isNodeInForest n xs

任何帮助将不胜感激!

Answer 1

看起来您已经有了一个合理的开始。

我假设您希望 deleteNode 获取一个森林并返回同一森林，但删除了指定的节点。在这种情况下，

if isNodeInTree n x then ... else deleteNode n xs

你刚刚扔掉了x。你可能不是故意的。

... else x : deleteNode n xs

将那棵树保留在森林中，这可能就是您的意图。

此外，在deleteNodeFromTree中:

if identifier c == n then [] ...

也许此时您想返回该节点的所有子节点？ (所以它们都成为根节点。)

这些是对我来说唯一突出的事情。看看它会带你去哪里......