F#：n元树提取单个元素并保存计算

type NTree<'a> =
    | Node of 'a * NTree<'a> list

let rec extractElement (tree: NTree<'a>) (index: int) : 'a option =
    match tree with
    | Node (value, children) ->
        if index = 0 then Some value
        else
            let rec extractFromChildren (index: int) (children: NTree<'a> list) : 'a option =
                match children with
                | [] -> None
                | child::rest ->
                    match extractElement child index with
                    | Some result -> Some result
                    | None -> extractFromChildren (index - 1) rest
            extractFromChildren (index - 1) children

let tree = Node (1, [
    Node (2, []);
    Node (3, [
        Node (4, []);
        Node (5, []);
    ]);
    Node (6, []);
])

let result = extractElement tree 2
match result with
| Some value -> printfn "Extracted element: %d" value
| None -> printfn "Invalid index"

Extracted element: 3