diff --git a/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_1.py b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_1.py
new file mode 100644
index 0000000..0de303b
--- /dev/null
+++ b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_1.py	
@@ -0,0 +1,45 @@
+# BFSで実装
+class Codec:
+    def serialize(self, root):
+        """Encodes a tree to a single string.
+
+        :type root: TreeNode
+        :rtype: str
+        """
+        if not root:
+            return ""
+        serial_tree = []
+        node_queue = deque([root])
+        while node_queue:
+            node = node_queue.popleft()
+            if not node:
+                serial_tree.append("null")
+                continue
+            serial_tree.append(str(node.val))
+            node_queue.append(node.left)
+            node_queue.append(node.right)
+        return ",".join(serial_tree)
+
+    def deserialize(self, data):
+        """Decodes your encoded data to tree.
+
+        :type data: str
+        :rtype: TreeNode
+        """
+        if not data:
+            return None
+        data_queue = deque(list(data.split(",")))
+        value = data_queue.popleft()
+        root = TreeNode(value)
+        node_queue = deque([root])
+        while data_queue:
+            left_value = data_queue.popleft()
+            right_value = data_queue.popleft()
+            node = node_queue.popleft()
+            if not left_value == "null":
+                node.left = TreeNode(left_value)
+                node_queue.append(node.left)
+            if not right_value == "null":
+                node.right = TreeNode(right_value)
+                node_queue.append(node.right)
+        return root
diff --git a/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_2.py b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_2.py
new file mode 100644
index 0000000..af9b65a
--- /dev/null
+++ b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_2.py	
@@ -0,0 +1,47 @@
+# DFSで実装
+class Codec:
+    def serialize(self, root):
+        """Encodes a tree to a single string.
+
+        :type root: TreeNode
+        :rtype: str
+        """
+
+        serial_tree = []
+
+        def helper_serialize(node):
+            if not node:
+                serial_tree.append("null")
+                return
+            serial_tree.append(str(node.val))
+            helper_serialize(node.left)
+            helper_serialize(node.right)
+
+        helper_serialize(root)
+        return ",".join(serial_tree)
+
+    def deserialize(self, data):
+        """Decodes your encoded data to tree.
+
+        :type data: str
+        :rtype: TreeNode
+        """
+
+        def helper_deserialize(data):
+            value = data.popleft()
+            if value == "null":
+                return None
+            node = TreeNode(value)
+            node.left = helper_deserialize(data)
+            node.right = helper_deserialize(data)
+            return node
+
+
+        root = helper_deserialize(deque(data.split(",")))
+        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/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_3.py b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_3.py
new file mode 100644
index 0000000..ea6058a
--- /dev/null
+++ b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_3.py	
@@ -0,0 +1,45 @@
+class Codec:
+    def serialize(self, root):
+        """Encodes a tree to a single string.
+
+        :type root: TreeNode
+        :rtype: str
+        """
+
+        serial_tree = []
+
+        def helper_serialize(node):
+            if not node:
+                serial_tree.append("null")
+                return
+            serial_tree.append(str(node.val))
+            helper_serialize(node.left)
+            helper_serialize(node.right)
+
+        helper_serialize(root)
+        return ",".join(serial_tree)
+
+    def deserialize(self, data):
+        """Decodes your encoded data to tree.
+
+        :type data: str
+        :rtype: TreeNode
+        """
+
+        def helper_deserialize(data):
+            value = data.popleft()
+            if value == "null":
+                return None
+            node = TreeNode(value)
+            node.left = helper_deserialize(data)
+            node.right = helper_deserialize(data)
+            return node
+
+        root = helper_deserialize(deque(data.split(",")))
+        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/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_4.py b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_4.py
new file mode 100644
index 0000000..92baa43
--- /dev/null
+++ b/grind75_hard/02_297. Serialize and Deserialize Binary Tree/level_4.py	
@@ -0,0 +1,50 @@
+# NULL_SYMBOLを作成
+# helper_deserializeの引数をイテレータに変更
+# 変数名を修正
+class Codec:
+    NULL_SYMBOL = "#"
+
+    def serialize(self, root):
+        """Encodes a tree to a single string.
+
+        :type root: TreeNode
+        :rtype: str
+        """
+
+        serialized_tree = []
+
+        def helper_serialize(node):
+            if not node:
+                serialized_tree.append(self.NULL_SYMBOL)
+                return
+            serialized_tree.append(str(node.val))
+            helper_serialize(node.left)
+            helper_serialize(node.right)
+
+        helper_serialize(root)
+        return ",".join(serialized_tree)
+
+    def deserialize(self, data):
+        """Decodes your encoded data to tree.
+
+        :type data: str
+        :rtype: TreeNode
+        """
+
+        def helper_deserialize(data):
+            value = next(data)
+            if value == self.NULL_SYMBOL:
+                return None
+            node = TreeNode(value)
+            node.left = helper_deserialize(data)
+            node.right = helper_deserialize(data)
+            return node
+
+        root = helper_deserialize(iter(data.split(",")))
+        return root
+
+
+# Your Codec object will be instantiated and called as such:
+# ser = Codec()
+# deser = Codec()
+# ans = deser.deserialize(ser.serialize(root))