f#上二叉树的前序遍历

type BinaryTree<'T> =
    | Empty
    | Node of 'T * BinaryTree<'T> * BinaryTree<'T>

let rec preorderTraversal (tree: BinaryTree<'T>) =
    match tree with
    | Empty -> []
    | Node(value, left, right) ->
        [value] @ (preorderTraversal left) @ (preorderTraversal right)

let tree =
    Node(1,
        Node(2,
            Node(4, Empty, Empty),
            Node(5, Empty, Empty)),
        Node(3,
            Node(6, Empty, Empty),
            Node(7, Empty, Empty)))

let result = preorderTraversal tree
printfn "%A" result