将树转换为列表 [英] Convert tree to list
问题描述
如何将树转换为列表: 到目前为止,我有以下代码,但给出了几个问题:
How can I convert a tree to a list: So far I have the following code but its giving several issues:
type 'a tree = Lf | Br of 'a * 'a tree * 'a tree;;
let rec elementRight xs = function
| LF ->false
| Br(m,t1,t2) -> if elementRight xs t1 = false then t1::xs else element xs t1;; //cannot find element
let rec elementLeft xs = function
| LF ->false
| Br(m,t1,t2) -> if elementLeft xs t2 = false then t2::xs else element xs t2 ;; //cannot find element
let rec element xs = function
| LF ->[]
| Br(m,t1,t2) -> xs::(elementRight xs t1)::(elementRight xs t2)::(elementLeft xs t1)::(elementLeft xs t2);;
推荐答案
您的代码存在许多问题:
There are a number of problems with your code:
-
您不应该在行尾使用
;;
(我猜这意味着您要复制并粘贴到repl中,您应该真正使用fsx文件,而应使用"send to repl ).
You shouldn't have
;;
at the end of lines (I'm guessing this means you're copy and pasting into the repl, you should really use an fsx file instead and use "send to repl").
此:| LF ->false
返回布尔值,而此:| Br(m,t1,t2) -> if elementRight xs t1 = false then t1::xs else element xs t1
返回'a list
.一个表达式只能有一种类型,因此返回两种是编译错误.我猜你真正的意思是让叶子返回[]
,然后在分支情况下检查空列表,如下所示:
This: | LF ->false
is returning a bool, while this: | Br(m,t1,t2) -> if elementRight xs t1 = false then t1::xs else element xs t1
is returning an 'a list
. An expression can only have one type, so returning two is a compile error. I'm guessing what you really are meaning to do is have the leaf return []
and then check for empty list in your branch case something like this:
let rec elementRight xs = function
| LF ->[]
| Br(m,t1,t2) -> if elementRight xs t1 = List.empty then t1::xs else element xs t1
3.当使用相互递归函数时,您需要对所有声明使用and
关键字,但第一个是这样的:
3 . when using mutually recursive functions you need to use the and
keyword for all declarations but the first like this:
let rec elementRight xs = function
...
and elementLeft xs = function
...
and element xs = function
...
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