前順の走査で二分木を文字列にして、文字列から同じ木に組み直す

def serialize(root):
    out = []

    def walk(n):
        if n is None:
            out.append("#")
            return
        out.append(str(n.val))
        walk(n.left)
        walk(n.right)

    walk(root)
    return ",".join(out)


def deserialize(data):
    items = data.split(",")
    it = iter(items)

    def build():
        item = next(it)
        if item == "#":
            return None
        n = TreeNode(int(item))
        n.left = build()
        n.right = build()
        return n

    return build()


def tree_codec(root):
    text = serialize(root)
    back = deserialize(text)
    return serialize(back)

function serialize(root) {
  const out = [];
  const walk = (node) => {
    if (node === null) {
      out.push("#");
      return;
    }
    out.push(String(node.val));
    walk(node.left);
    walk(node.right);
  };
  walk(root);
  return out.join(",");
}

function deserialize(data) {
  const items = data.split(",");
  let i = 0;
  const build = () => {
    const item = items[i++];
    if (item === "#") {
      return null;
    }
    const node = new TreeNode(
      Number(item)
    );
    node.left = build();
    node.right = build();
    return node;
  };
  return build();
}

function treeCodec(root) {
  const text = serialize(root);
  const back =
    deserialize(text);
  return serialize(back);
}

import (
	"strconv"
	"strings"
)

func serialize(
	root *TreeNode,
) string {
	out := []string{}
	var walk func(*TreeNode)
	walk = func(n *TreeNode) {
		if n == nil {
			out = append(out, "#")
			return
		}
		out = append(out,
			strconv.Itoa(n.Val))
		walk(n.Left)
		walk(n.Right)
	}
	walk(root)
	return strings.Join(out, ",")
}

func deserialize(
	data string,
) *TreeNode {
	items := strings.Split(
		data, ",")
	i := 0
	var build func() *TreeNode
	build = func() *TreeNode {
		item := items[i]
		i++
		if item == "#" {
			return nil
		}
		v, _ := strconv.Atoi(item)
		n := &TreeNode{Val: v}
		n.Left = build()
		n.Right = build()
		return n
	}
	return build()
}

func treeCodec(
	root *TreeNode,
) string {
	text := serialize(root)
	back := deserialize(text)
	return serialize(back)
}