Function 二叉树的SML和

我必须写一个函数，它可以找到二叉树中所有元素的总和

这就是它的定义（树）：

我不知道该怎么做。我承认我是SML的初学者

例如：

sumTree（节点（节点（叶1、3、叶2）、7、叶4））

应该返回17。sumTree是函数的名称

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谢谢你的帮助

我希望你能花点时间理解这一点。如果您对此有任何疑问，请告诉我：

(*
 * This is a recursive datatype, because the Node constructor contains two
 * elements of type `tree`, the type which we're currently defining.
 *)
datatype 'a tree =
    Leaf of 'a
  | Node of 'a tree * 'a * 'a tree

(*
 * Notice how the structure of the `sumTree` function follows the structure
 * of the `tree` datatype. But `tree` is a recursive datatype, which means
 * that `sumTree` will probably be a recursive function. This is called
 * structural recursion. We recurse accordingly to the structure of the type.
 *)
fun sumTree (Leaf n) = n
  | sumTree (Node (left, n, right)) = (sumTree left) + n + (sumTree right)

基本思想是解决简单案例（

Leaf

）的问题，而在复杂案例（

Node

）中，则依赖于简单案例的解决方案

(*
 * This is a recursive datatype, because the Node constructor contains two
 * elements of type `tree`, the type which we're currently defining.
 *)
datatype 'a tree =
    Leaf of 'a
  | Node of 'a tree * 'a * 'a tree

(*
 * Notice how the structure of the `sumTree` function follows the structure
 * of the `tree` datatype. But `tree` is a recursive datatype, which means
 * that `sumTree` will probably be a recursive function. This is called
 * structural recursion. We recurse accordingly to the structure of the type.
 *)
fun sumTree (Leaf n) = n
  | sumTree (Node (left, n, right)) = (sumTree left) + n + (sumTree right)