(exercises) implement tree_of_string function for problem 70A (#3135)

* feat: implement tree_of_string function for problem 70A

* break up into three REPL statements

---------

Co-authored-by: sabine <6594573+sabine@users.noreply.github.com>

Author: zlh20040308

data/exercises/070A_create_mtree.md

@@ -19,13 +19,37 @@ type 'a mult_tree = T of 'a * 'a mult_tree list
   let rec add_string_of_tree buf (T (c, sub)) =
     Buffer.add_char buf c;
     List.iter (add_string_of_tree buf) sub;
-    Buffer.add_char buf '^'
-  let string_of_tree t =
+    Buffer.add_char buf '^';;
+val add_string_of_tree : Buffer.t -> char mult_tree -> unit = <fun>
+
+# let string_of_tree t =
     let buf = Buffer.create 128 in
     add_string_of_tree buf t;
     Buffer.contents buf;;
-val add_string_of_tree : Buffer.t -> char mult_tree -> unit = <fun>
+
 val string_of_tree : char mult_tree -> string = <fun>
+
+# let tree_of_string s =
+    let rec parse_node chars =
+      match chars with
+      | [] -> failwith "Unexpected end of input (expecting node)"
+      | c :: rest ->
+          let (children, rest') = parse_children rest in
+          (T (c, children), rest')
+    and parse_children chars =
+      match chars with
+      | [] -> failwith "Unexpected end of input (expecting ^)"
+      | '^' :: rest -> ([], rest)
+      | _ ->
+          let (child, rest') = parse_node chars in
+          let (siblings, rest'') = parse_children rest' in
+          (child :: siblings, rest'')
+    in
+    let (tree, remaining) = parse_node (List.of_seq (String.to_seq s)) in
+    match remaining with
+    | [] -> tree
+    | _ -> failwith "Extra input after tree";;
+val tree_of_string : string -> char mult_tree = <fun>
 ```
 
 # Statement