Solved Arai60/104. Maximum Depth of Binary Tree

Python3/104. Maximum Depth of Binary Tree.md

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+## Step 1. Initial Solution
+
+- BFSで実装できる
+
+```python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        unprocessed_nodes: List[Tuple[TreeNode, int]] = deque([(root, 1)])
+        max_depth: int = 0
+        while unprocessed_nodes:
+            node, depth = unprocessed_nodes.popleft()
+            if node is None:
+                continue
+            max_depth = max(max_depth, depth)
+            unprocessed_nodes.append((node.left, depth + 1))
+            unprocessed_nodes.append((node.right, depth + 1))
+        return max_depth
+```
+
+### Complexity Analysis
+
+- 時間計算量：O(n)
+    - 全ノードを見る
+    - ループ内の処理は定数時間
+- 空間計算量：O(n)
+
+## Step 2. Alternatives
+
+- 再帰を使った方法
+    - とてもシンプルに書ける
+
+```python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
+```
+
+- 他にもいくつか読んだが基本的にはBFSかDFSか、Depthをどう保持するか、Noneのチェックをどこで行うか、変数を新しく置くか、くらいの差しかなさそう
+
+## Step 3. Final Solution
+
+- 変数名が少し長いがこのくらいが分かりやすいギリギリのラインか
+
+```python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        remaining_nodes_and_depths: List[Tuple[TreeNode, int]] = [(root, 1)]
+        max_depth: int = 0
+        while remaining_nodes_and_depths:
+            node, depth = remaining_nodes_and_depths.pop()
+            if node is None:
+                continue
+            max_depth = max(max_depth, depth)
+            remaining_nodes_and_depths.append((node.left, depth + 1))
+            remaining_nodes_and_depths.append((node.right, depth + 1))
+        return max_depth
+```