103. Binary Tree Zigzag Level Order Traversal

103/memo.md

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+# 103. Binary Tree Zigzag Level Order Traversal
+- [リンク](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/)
+- https://github.com/mamo3gr/arai60/blob/103_binary-tree-zigzag-level-order-traversal/103_binary-tree-zigzag-level-order-traversal/memo.md
+- リストを「逆順」に扱う方法（用途で選ぶ）
+    - `xs.reverse()`：破壊的（in-place）。リスト自体を反転して良いなら最有力
+    - `reversed(xs)`：非破壊の逆順イテレータ。`for` で回すだけならこれが軽い（コピー不要）
+    - `list(reversed(xs))` / `xs[::-1]`：**非破壊**で「反転した新しいリスト」が欲しいとき（どちらも \(O(n)\) コピー）
+    - 手動で `for i in range(len(xs)-1, -1, -1)`：特殊な制御が必要なときだけ（可読性は落ちがち）
+- 勉強のためにdfs+再帰も書く

103/sol1_dfs.py

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+from typing import Optional, List
+
+
+# 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 zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        levels: List[List[int]] = []
+
+        def traverse(node: Optional[TreeNode], depth: int) -> None:
+            if node is None:
+                return
+
+            while len(levels) <= depth:
+                levels.append([])
+
+            if depth % 2 == 0:
+                levels[depth].append(node.val)
+            else:
+                levels[depth].insert(0, node.val)
+
+            traverse(node.left, depth + 1)
+            traverse(node.right, depth + 1)
+
+        traverse(root, 0)
+        return levels

103/sol2_dfs_recursion.py

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+from typing import Optional, List
+
+
+# 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 zigzagLevelOrder(self, root) -> List[List[int]]:
+        zigzag_level_order = []
+
+        def traverse(node, depth: int):
+            if node is None:
+                return
+
+            while len(zigzag_level_order) <= depth:
+                zigzag_level_order.append([])
+
+            if depth % 2 == 0:
+                zigzag_level_order[depth].append(node.val)
+            else:
+                zigzag_level_order[depth].insert(0, node.val)
+
+            traverse(node.left, depth + 1)
+            traverse(node.right, depth + 1)
+
+        traverse(root, 0)
+        return zigzag_level_order