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+## Step1
+
+- 最初に思いついた解法
+- depthを中でも参照するために、returnでdepth, max_depthの両方を返してしまったが、そもそもclassなのでself.で再帰関数を定義すればよかった
+- 最初はreturn depth_search(depth, root, max_depth)[1]だったが見にくい気がして直す
+    - なぜか実行速度も45ms→33msに
+
+```python
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        def depth_search(depth, current_node, max_depth):
+            depth += 1
+            if not current_node:
+                return depth, max_depth
+            max_depth = max(depth, max_depth)
+            depth, max_depth = depth_search(depth, current_node.left, max_depth)
+            depth -= 1
+            depth, max_depth = depth_search(depth, current_node.right, max_depth)
+            depth -= 1
+            return depth, max_depth
+        
+        depth = 0
+        max_depth = 0
+        depth, max_depth = depth_search(depth, root, max_depth)
+        return max_depth
+```
+
+## Step2
+
+- [ahayashiさん](https://github.com/hayashi-ay/leetcode/pull/22/files)のをみた。よく考えたら、再帰ならdepthは持たなくても良い
+
+```python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        max_depth = max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
+        return max_depth
+```
+
+- 上のは簡潔だけど、left_depthとright_depthで一旦分けといた方がいいのか？迷う
+    - Step3では分けることに
+- BFS順に見ていくやつも実装してみた
+    - 1回目（失敗作）が上、直したのが下
+    
+    ```python
+    python
+    class Solution:
+        def maxDepth(self, root: Optional[TreeNode]) -> int:
+            if not root:
+                return 0
+            depth = 1
+            current_node = [root]
+            while current_node:
+                depth += 1
+                next_node = []
+                if current_node.left:
+                    next_node.append(current_node.left)
+                if current_node.right:
+                    next_node.append(curent_node.right)
+                current_node = next_node
+            return depth
+    ```
+    
+
+```python
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        current_nodes = [root]
+        depth = 0
+        while current_nodes:
+            depth += 1
+            next_nodes = []
+            for node in current_nodes:
+                if node.left:
+                    next_nodes.append(node.left) 
+                if node.right:
+                    next_nodes.append(node.right) 
+            current_nodes = next_nodes
+        return depth
+```
+
+- DFS順のやつも実装、StackでDFSを実装するのは初めてなので練習
+    - 割とすぐできた、うっかりappend(node.right, depth+1)とやってしまい一瞬あれ、となってたけど
+    - [これ](https://github.com/shining-ai/leetcode/pull/21/files)を見る限り、node_depthという変数名よりstack_node_depthとstackであることを明示した方がいいのかな
+        - 迷う
+
+```python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        depth = 1
+        max_depth = 1
+        node_depth = [(root, depth)]
+        while node_depth:
+            node, depth = node_depth.pop()
+            max_depth = max(max_depth, depth)
+            if node.right:
+                node_depth.append((node.right, depth+1))
+                max_depth = max(max_depth, depth)
+            if node.left:
+                node_depth.append((node.left, depth+1))
+                max_depth = max(max_depth, depth)
+        return max_depth
+```
+
+## Step3
+
+- BFS（これが一番詰まる）
+
+```python
+def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        depth = 0
+        current_nodes = [root]
+        while current_nodes:
+            depth += 1
+            next_nodes = []
+            for node in current_nodes:
+                if node.left:
+                    next_nodes.append(node.left)
+                if node.right:
+                    next_nodes.append(node.right)
+                current_nodes = next_nodes
+        return depth
+```
+
+- DFS
+
+よく考えたら、maxを取るのはpop()する時だけでOK
+
+```python
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        node = root
+        depth = 1
+        stack_node_depth = [(node, depth)]
+        max_depth = 1
+        while stack_node_depth:
+            node, depth = stack_node_depth.pop()
+            max_depth = max(max_depth, depth)
+            if node.right:
+                stack_node_depth.append((node.right, depth+1))
+            if node.left:
+                stack_node_depth.append((node.left, depth+1))
+        return max_depth
+```
+
+- 再帰（これは簡単）
+    - なぜかmax_depth = max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1と一気に書くよりだいぶ遅い(一気に書くのは27ms、下のは40ms)
+
+```python
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        left_depth = self.maxDepth(root.left)
+        right_depth = self.maxDepth(root.right)
+        return max(left_depth, right_depth) + 1
+```