diff --git a/arai60/maximum-depth-of-binary-tree/README.md b/arai60/maximum-depth-of-binary-tree/README.md
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+## 考察
+- 過去に解いたことあり
+- 方針
+    - DFS
+        - 左部分木と右部分木のdepthのmaxを取って1を追加すれば良さそう
+        - 帰りがけで計算したいので再帰で書きたいところ
+        - 再帰の深さが最大10^4になるのが気になるが、recursionlimitが550000に設定されているので大丈夫そう
+    - BFS
+        - 深さ(レベル)別に見ていけば、最後に探索するノードの深さが答え
+- Step1ではDFSで実装してみる
+
+## Step1
+- 再帰DFSで実装
+- time: O(n), space: O(n)
+
+## Step2
+- BFSでも実装
+    - キューのところをスタックにすれば非再帰DFSにもなる
+    - time: O(n), space: O(n)
+- 他の人のPRを検索
+    - 行きと帰りのラベルを付けてスタックに積めば、帰りがけの計算も非再帰で表現可能
+        - https://discord.com/channels/1084280443945353267/1201211204547383386/1228400327800127648
+
+## Step3
+- 1回目: 28s
+- 2回目: 28s
+- 3回目: 26s
+
+## Step4
+- キューにdepthを入れないversionのBFSを追加
diff --git a/arai60/maximum-depth-of-binary-tree/step1.py b/arai60/maximum-depth-of-binary-tree/step1.py
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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
diff --git a/arai60/maximum-depth-of-binary-tree/step2.py b/arai60/maximum-depth-of-binary-tree/step2.py
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+++ b/arai60/maximum-depth-of-binary-tree/step2.py
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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        nodes_to_visit = deque([(root, 1)])
+        max_depth = 0
+
+        while nodes_to_visit:
+            node, depth = nodes_to_visit.popleft()
+            if not node: continue
+            max_depth = max(max_depth, depth)
+            nodes_to_visit.append((node.left, depth + 1))
+            nodes_to_visit.append((node.right, depth + 1))
+
+        return max_depth
diff --git a/arai60/maximum-depth-of-binary-tree/step3.py b/arai60/maximum-depth-of-binary-tree/step3.py
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+++ b/arai60/maximum-depth-of-binary-tree/step3.py
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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
diff --git a/arai60/maximum-depth-of-binary-tree/step4.py b/arai60/maximum-depth-of-binary-tree/step4.py
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+++ b/arai60/maximum-depth-of-binary-tree/step4.py
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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+
+        max_depth = 0
+        nodes_to_visit = deque([root])
+        while nodes_to_visit:
+            max_depth += 1
+            level_size = len(nodes_to_visit)
+            for _ in range(level_size):
+                node = nodes_to_visit.popleft()
+                if node.left:
+                    nodes_to_visit.append(node.left)
+                if node.right:
+                    nodes_to_visit.append(node.right)
+
+        return max_depth