diff --git a/arai60/phase1.py b/arai60/phase1.py
new file mode 100644
index 0000000..2f578e2
--- /dev/null
+++ b/arai60/phase1.py
@@ -0,0 +1,17 @@
+# 再帰の解き方
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        max_depth = 0
+        def traverse_nodes(node, depth):
+            # node: the node you concentrate on
+            # depth: the depth of the node you concentrate on
+            nonlocal max_depth
+            if node == None:
+                return
+            if max_depth < depth:
+                max_depth = depth
+            traverse_nodes(node.left, depth + 1)
+            traverse_nodes(node.right, depth + 1)
+
+        traverse_nodes(root, 1)
+        return max_depth
diff --git a/arai60/phase2.py b/arai60/phase2.py
new file mode 100644
index 0000000..7575d71
--- /dev/null
+++ b/arai60/phase2.py
@@ -0,0 +1,50 @@
+"""
+Reference
+fhiyo: https://github.com/fhiyo/leetcode/pull/23/files phase1の段階で別の関数挟まずに書き終えてて凄すぎてびっくりしてしまった。関数プログラミングっぽい書き方
+nittoco: https://github.com/nittoco/leetcode/pull/14/files
+rossy0213: https://github.com/rossy0213/leetcode/pull/10
+Mike0121: https://github.com/Mike0121/LeetCode/pull/6#discussion_r1590279702
+traverse_nodesに型ヒントを追加
+"""
+# 簡潔
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        return max(self.maxDepth(root.right), self.maxDepth(root.left)) + 1
+
+
+
+# phase1の直し, 型ヒントを追加, node == Noneをnode is Noneに
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        max_depth = 0
+        def traverse_nodes(node: Optional[TreeNode], depth: int) -> None:
+            # node: the node you concentrate on
+            # depth: the depth of the node you concentrate on
+            nonlocal max_depth
+            if node is None:
+                return
+            if max_depth < depth:
+                max_depth = depth
+            traverse_nodes(node.left, depth + 1)
+            traverse_nodes(node.right, depth + 1)
+
+        traverse_nodes(root, 1)
+        return max_depth
+
+# 一応再帰以外でも
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        max_depth = 0
+        stack = [(root, 1)] 
+        while stack:
+            current_node, depth_so_far = stack.pop()
+            if current_node is None:
+                continue
+            if max_depth < depth_so_far:
+                max_depth = depth_so_far
+            stack.append((current_node.right, depth_so_far + 1))
+            stack.append((current_node.left, depth_so_far + 1))
+
+        return max_depth
diff --git a/arai60/phase3.py b/arai60/phase3.py
new file mode 100644
index 0000000..44544f1
--- /dev/null
+++ b/arai60/phase3.py
@@ -0,0 +1,16 @@
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        max_depth = 0
+        def traverse(node: Optional[TreeNode], depth: int) -> None:
+            # node: the node you concentrate on
+            # depth: the depth of the node you concentrate on
+            nonlocal max_depth
+            if node is None:
+                return
+            if max_depth < depth:
+                max_depth = depth
+            traverse(node.right, depth + 1)
+            traverse(node.left, depth + 1)
+        
+        traverse(root, 1)
+        return max_depth