Question

data BinTree a = Empty | Node a (BinTree a) (BinTree a)
    deriving (Show)

我正试图找出一种方式来显示二叉树，使得对于每个级别我在树中下来，我想在节点名称旁边添加一个额外的*并将它们全部分开\n。

例如：

let x = Node "Parent" (Node "childLeft" (Node "grandChildLeftLeft" Emp Emp) Emp) (Node "childRight" Emp Emp)
putStrLn $ displayTree x

应该返回：

"Parent"
*"childLeft"
**"grandChildLeftLeft"
*"childRight"

我的功能（最多只打印一个*）：

displayTree :: Show a => BinTree a -> String
displayTree Emp = ""
displayTree (Node head Emp Emp) = (show head)
displayTree (Node head left Emp) = (show head) ++ "\n*" ++ displayTree left
displayTree (Node head Emp right) = (show head) ++ "\n*" ++ displayTree right
displayTree (Node head left right) = (show head) ++ "\n*" ++ displayTree left ++ "\n*" ++ displayTree right

我的displayTree功能会打印出来：

"Parent"
*"childLeft"
*"grandChildLeftLeft"
*"childRight"

我希望"grandChildLeftLeft"旁边有**而不只是*。

有什么建议吗？

注意：我不想更改传递给函数的参数，因此它应保持为displayTree :: Show a => BinTree a -> String

Answer 1

我认为这就是你想要的：

module Main (main) where

data BinTree a = Empty | Node a (BinTree a) (BinTree a)
    deriving (Show)

showTree :: Show a => BinTree a -> Int -> String
showTree (Empty) _ = []
showTree (Node t l r) n = replicate n '*' ++ show t ++ "\n" ++ showTree l (n+1) ++ showTree r (n+1)

main :: IO ()
main = do
    let x = Node "Parent" (Node "childLeft" (Node "grandChildLeftLeft" Empty Empty) Empty) (Node "childRight" Empty Empty)
    putStrLn $ showTree x 0

请注意累加器n，它会在每次递归调用时更改缩进级别。

http://ideone.com/lphCoV

"Parent"
*"childLeft"
**"grandChildLeftLeft"
*"childRight"

Answer 2

为什么不将深度传递给displayTree函数？

displayTree :: Show a => BinTree a -> String 
displayTree = displayTree' ""

displayTree' str Emp = ""
displayTree' str (Node head Emp Emp) = str ++ (show head)
displayTree' str (Node head left Emp) = str ++ (show head) ++ "\n" ++ displayTree' (str ++ "*") left
displayTree' str (Node head Emp right) = str ++ (show head) ++ "\n" ++ displayTree' (str ++ "*") right
displayTree' str (Node head left right) = str ++ (show head) ++ "\n" ++ displayTree' (str ++ "*") left ++ "\n" ++ displayTree (str ++ "*") right

此外，这里重构的是更具可读性：

displayTree :: Show a => BinTree a -> String 
displayTree = displayTree' ""

displayTree' str Empty = ""
displayTree' str (Node h l r) = hstr ++ lstr ++ rstr
    where
        hstr = str ++ (show head) ++ "\n"
        lstr = makeTreeStr l
        rstr = makeTreeStr r
        makeTreeStr Empty = ""
        makeTreeStr tree = displayTree' (str ++ "*") tree ++ "\n"

在Haskell中显示二叉树

2 个答案: