Haskell,如何计算树的所有节点(模式匹配)

Question

问题出在标题:)

我的代码:

data tree a = Leaf a | Node (tree a) (tree a)

treeo = Node((Node(Leaf 1)(Node (Leaf 10)(Leaf 11))))
                (Node(Leaf 12)(Leaf 13))

-- count all leafs: 
lcounter(Leaf a) = 1
lcounter(Node a b)= lcounter a + lcounter b
-- count all nodes?:

Answer 1

首先注意数据类型定义应该从大写开始:

data Tree a = Leaf a | Node (Tree a) (Tree a)

要统计所有节点,它几乎就是你所做的,一个简单的模式匹配树数据构造函数:

countNodes :: Tree a -> Int
countNodes (Leaf a) = 0
countNodes (Node left right) = 1 + countNodes left + countNodes right

用你的例子:

let tree = Node((Node (Leaf 1) (Node (Leaf 10) (Leaf 11))))(Node (Leaf 12) (Leaf 13))
countNodes tree -- 4