diff --git a/0297.Serialize-and-Deserialize-Binary-Tree/memo.md b/0297.Serialize-and-Deserialize-Binary-Tree/memo.md
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+# 297. Serialize and Deserialize Binary Tree
+
+## step1
+
+37mぐらいかかった。確実に解けると思ったが、スラスラと書くことができなかった。
+
+幅優先探索で解くことにした。
+
+間違えた点：
+- delimをつけずに間違えた(数字が2桁だとdecodeできない)
+- deserializeでキューになぜかNoneを入れた
+
+
+## step2
+
+https://support.leetcode.com/hc/en-us/articles/32442719377939-How-to-create-test-cases-on-LeetCode#h_01J5EGREAW3NAEJ14XC07GRW1A
+
+こんなサイトがあることを知らなかった。level order traversalという名前がついていたんだった。
+
+探索順は同じだが、serializeした文字列がこちらの方が短い。nullの場合の処理が異なる。
+
+この場合、自身をresultに入れてから、子をresultに入れるとかける: step2
+
+こちらも書くのに20mぐらいかかった。
+
+書きながら注意したことで、以下は正しく動かない。
+
+```python
+child = node.left
+child = TreeNode(0)
+```
+
+これはPythonの変数がオブジェクトの参照であるためで、childが単に`TreeNode(0)`オブジェクトを指すようになる、という変更になる。
+
+## 他の解法
+
+LeetCodeの与えられ方に引きずられて、level-orderを最初に選択したが、DFSでも解くことができるはず。
+
+11mぐらいでかけた。
+
+Pre-orderとPost-orderは可能だが、In-orderは可能なのか？おそらく不可能だと思う。理由は、in-orderだとnull Nodeがある場合に根の位置が特定できないから。
+
+
+LeetCodeのSolutionを検索すると in-order と書かれているものがあったが、コードを読むと実際にはpre-orderであった。
+https://leetcode.com/problems/serialize-and-deserialize-binary-tree/solutions/6678162/in-order-traversal-simple-solution-by-th-822j/?envType=problem-list-v2&envId=rab78cw1
+
+
+
+## 他の人のコード
+
+https://github.com/hayashi-ay/leetcode/pull/74
+
+> 書いてみた感想として、探索だけならDFSもBFSもそこまで書きやすさは変わらないですが、Treeの構築が発生する場合はBFSおよびDFSのループだと書きにくいなと思いました。親ノードや左右どちらにつなぐかなどを考えないといけないので。
+
+確かに自分もBFSで書くので苦戦し、再帰DFSで書くと簡単にかけた
+
+自分はindexを使ったが、iteratorを使っている。例外で処理しようとしたが、nextの引数defaultを勧められている。nextの引数defaultは知らなかった。
+
+https://docs.python.org/3/library/functions.html#next
+
+https://github.com/shining-ai/leetcode/pull/62
+
+> イテレータを使う発想がありませんでした。
+
+自分も同じく。
+
+https://github.com/potrue/leetcode/pull/73
+
+## step3
+最も個人的に自然な pre-orderを書いておく
diff --git a/0297.Serialize-and-Deserialize-Binary-Tree/step1_level_order.py b/0297.Serialize-and-Deserialize-Binary-Tree/step1_level_order.py
new file mode 100644
index 0000000..503be73
--- /dev/null
+++ b/0297.Serialize-and-Deserialize-Binary-Tree/step1_level_order.py
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+import collections
+
+
+# Definition for a binary tree node.
+class TreeNode(object):
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Codec:
+    NO_NODE = "n"
+    DELIM = ","
+
+    def serialize(self, root: TreeNode | None) -> str:
+        """Encodes a tree to a single string."""
+        if root is None:
+            return self.NO_NODE
+
+        frontier = collections.deque([root])
+        result = []
+
+        while frontier:
+            node = frontier.popleft()
+            if node is None:
+                result.append(self.NO_NODE)
+                continue
+
+            result.append(str(node.val))
+            frontier.append(node.left)
+            frontier.append(node.right)
+
+        return self.DELIM.join(result)
+
+    def _create_node(self, c: str) -> TreeNode | None:
+        if c == self.NO_NODE:
+            return None
+        return TreeNode(int(c))
+
+    def deserialize(self, data: str) -> TreeNode | None:
+        """Decodes your encoded data to tree."""
+        if not data or data == self.NO_NODE:
+            return None
+
+        tokens = data.split(self.DELIM)
+
+        root = self._create_node(tokens[0])
+        frontier = collections.deque([root])
+
+        i = 1
+        while i < len(tokens):
+            node = frontier.popleft()
+
+            if node is None:
+                continue
+
+            node.left = self._create_node(tokens[i])
+            if node.left:
+                frontier.append(node.left)
+            i += 1
+
+            if i >= len(tokens):
+                continue
+
+            node.right = self._create_node(tokens[i])
+            if node.right:
+                frontier.append(node.right)
+            i += 1
+
+        return root
+
+
+# Your Codec object will be instantiated and called as such:
+# ser = Codec()
+# deser = Codec()
+# ans = deser.deserialize(ser.serialize(root))
diff --git a/0297.Serialize-and-Deserialize-Binary-Tree/step2_level_order_short.py b/0297.Serialize-and-Deserialize-Binary-Tree/step2_level_order_short.py
new file mode 100644
index 0000000..d5cdcaf
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+++ b/0297.Serialize-and-Deserialize-Binary-Tree/step2_level_order_short.py
@@ -0,0 +1,61 @@
+import collections
+
+
+# Definition for a binary tree node.
+class TreeNode(object):
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Codec:
+    NO_NODE = "n"
+    DELIM = ","
+
+    def serialize(self, root: TreeNode | None) -> str:
+        """Encodes a tree to a single string."""
+        if root is None:
+            return self.NO_NODE
+
+        result = [str(root.val)]
+        frontier = collections.deque([root])
+
+        while frontier:
+            node = frontier.popleft()
+            for child in (node.left, node.right):
+                if child is not None:
+                    result.append(str(child.val))
+                    frontier.append(child)
+                else:
+                    result.append(self.NO_NODE)
+
+        while result and result[-1] == self.DELIM:
+            result.pop()
+
+        return self.DELIM.join(result)
+
+    def deserialize(self, data: str) -> TreeNode | None:
+        """Decodes your encoded data to tree."""
+        if not data or data == self.NO_NODE:
+            return None
+
+        tokens = data.split(self.DELIM)
+
+        root = TreeNode(tokens[0])
+        frontier = collections.deque([root])
+
+        i = 1
+        while i < len(tokens):
+            node = frontier.popleft()
+
+            for child_name in ("left", "right"):
+                if tokens[i] != self.NO_NODE:
+                    child = TreeNode(tokens[i])
+                    setattr(node, child_name, child)
+                    frontier.append(child)
+                i += 1
+                if i >= len(tokens):
+                    break
+
+        return root
diff --git a/0297.Serialize-and-Deserialize-Binary-Tree/step2_postorder.py b/0297.Serialize-and-Deserialize-Binary-Tree/step2_postorder.py
new file mode 100644
index 0000000..29b6c24
--- /dev/null
+++ b/0297.Serialize-and-Deserialize-Binary-Tree/step2_postorder.py
@@ -0,0 +1,50 @@
+# Definition for a binary tree node.
+class TreeNode(object):
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Codec:
+    NO_NODE = "n"
+    DELIM = ","
+
+    def serialize(self, root: TreeNode | None) -> str:
+        """Encodes a tree to a single string."""
+        result = []
+
+        def traverse(node):
+            if node is None:
+                result.append(self.NO_NODE)
+                return
+
+            traverse(node.left)
+            traverse(node.right)
+            result.append(str(node.val))
+
+        traverse(root)
+        return self.DELIM.join(result)
+
+    def deserialize(self, data: str) -> TreeNode | None:
+        """Decodes your encoded data to tree."""
+        if not data or data == self.NO_NODE:
+            return None
+
+        tokens = data.split(self.DELIM)
+
+        def traverse() -> TreeNode | None:
+            if not tokens:
+                return None
+
+            token = tokens.pop()
+            if token == self.NO_NODE:
+                return None
+
+            node = TreeNode(token)
+            node.right = traverse()
+            node.left = traverse()
+
+            return node
+
+        return traverse()
diff --git a/0297.Serialize-and-Deserialize-Binary-Tree/step2_preorder.py b/0297.Serialize-and-Deserialize-Binary-Tree/step2_preorder.py
new file mode 100644
index 0000000..dcda725
--- /dev/null
+++ b/0297.Serialize-and-Deserialize-Binary-Tree/step2_preorder.py
@@ -0,0 +1,55 @@
+# Definition for a binary tree node.
+class TreeNode(object):
+    def __init__(self, x):
+        self.val = x
+        self.left = None
+        self.right = None
+
+
+class Codec:
+    NO_NODE = "n"
+    DELIM = ","
+
+    def serialize(self, root: TreeNode | None) -> str:
+        """Encodes a tree to a single string."""
+        result = []
+
+        def traverse(node):
+            if node is None:
+                result.append(self.NO_NODE)
+                return
+
+            result.append(str(node.val))
+            traverse(node.left)
+            traverse(node.right)
+
+        traverse(root)
+        return self.DELIM.join(result)
+
+    def deserialize(self, data: str) -> TreeNode | None:
+        """Decodes your encoded data to tree."""
+        if not data or data == self.NO_NODE:
+            return None
+
+        tokens = data.split(self.DELIM)
+
+        i = 0
+
+        def traverse():
+            nonlocal i
+
+            if i >= len(tokens):
+                return None
+
+            if tokens[i] == self.NO_NODE:
+                i += 1
+                return
+
+            node = TreeNode(tokens[i])
+            i += 1
+            node.left = traverse()
+            node.right = traverse()
+
+            return node
+
+        return traverse()
diff --git a/0297.Serialize-and-Deserialize-Binary-Tree/step3_preorder.py b/0297.Serialize-and-Deserialize-Binary-Tree/step3_preorder.py
new file mode 100644
index 0000000..ddd4dde
--- /dev/null
+++ b/0297.Serialize-and-Deserialize-Binary-Tree/step3_preorder.py
@@ -0,0 +1,50 @@
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+
+class Codec:
+    NO_NODE = "n"
+    DELIM = ","
+
+    def serialize(self, root: TreeNode | None) -> str:
+        """Encodes a tree to a single string."""
+        result = []
+
+        def traverse(node):
+            if node is None:
+                result.append(self.NO_NODE)
+                return
+            result.append(str(node.val))
+            traverse(node.left)
+            traverse(node.right)
+
+        traverse(root)
+        while result and result[-1] == self.NO_NODE:
+            result.pop()
+
+        return self.DELIM.join(result)
+
+    def deserialize(self, data: str) -> TreeNode | None:
+        """Decodes your encoded data to tree."""
+        if not data or data == self.NO_NODE:
+            return None
+
+        tokens = data.split(self.DELIM)
+        token_iterator = iter(tokens)
+
+        def traverse():
+            token = next(token_iterator, None)
+            if token is None or token == self.NO_NODE:
+                return None
+
+            node = TreeNode(int(token))
+            node.left = traverse()
+            node.right = traverse()
+
+            return node
+
+        return traverse()