103 binary tree zigzag level order traversal medium

103_binary_tree_zigzag_level_order_traversal_medium/README.md

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+# 問題へのリンク
+[103. Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)
+
+# 言語
+Python
+
+# 問題の概要
+
+
+# 自分の解法
+
+## step1
+
+```python
+class Solution:
+    def zigzagLevelOrder(self, root) -> list[list[int]]:
+        if not root:
+            return []
+        LEFT_TO_RIGHT = 1
+        RIGHT_TO_LEFT = -1
+        direction = LEFT_TO_RIGHT
+        nodes = [root]
+        zigzag_level_nodes = []
+        while nodes:
+            node_vals = []
+            next_level_nodes = []
+            while nodes:
+                node = nodes.pop()
+                node_vals.append(node.val)
+                if direction == LEFT_TO_RIGHT:
+                    if node.left:
+                        next_level_nodes.append(node.left)
+                    if node.right:
+                        next_level_nodes.append(node.right)
+                else:
+                    if node.right:
+                        next_level_nodes.append(node.right)
+                    if node.left:
+                        next_level_nodes.append(node.left)
+
+            zigzag_level_nodes.append(node_vals)
+            nodes = next_level_nodes
+            direction *= -1
+        return zigzag_level_nodes
+```
+
+- 時間計算量：`O(n)`
+- 空間計算量：`O(n)`
+
+## step2
+
+```python
+from collections import deque
+
+
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        if not root:
+            return []
+        zigzag_level_nodes = []
+        nodes = deque([(root, 0)])
+        while nodes:
+            next_nodes = deque([])
+            while nodes:
+                node, level = nodes.popleft()
+                while len(zigzag_level_nodes) <= level:
+                    zigzag_level_nodes.append(deque([]))
+
+                # left to right
+                if level % 2 == 0:
+                    zigzag_level_nodes[level].append(node.val)
+                # right to left
+                else:
+                    zigzag_level_nodes[level].appendleft(node.val)
+
+                if node.left:
+                    next_nodes.append((node.left, level + 1))
+                if node.right:
+                    next_nodes.append((node.right, level + 1))
+
+            nodes = next_nodes
+
+        return [list(deq) for deq in zigzag_level_nodes]
+```
+
+
+- BFSで各レベルごとに走査していくのはstep1と同じ。だが、走査するのは左から順に行い、値の追加だけzigzagになるようにする。
+
+## step3
+
+
+レベルごとに走査を行う方法（`step3_1.py`）。直感的だが、ネストが深くなる。
+リストを再代入していくのもあまり良くないかも。
+
+```python
+from collections import deque
+
+
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        zigzag_node_vals: list[deque[int]] = [deque([])]  # deque
+        nodes = [root]
+        next_level_nodes = []
+        level = 0
+
+        while nodes:
+            for node in nodes:
+                if node is None:
+                    continue
+                next_level_nodes.append(node.left)
+                next_level_nodes.append(node.right)
+                # left to right; FIFO
+                if level % 2 == 0:
+                    zigzag_node_vals[level].append(node.val)
+                else:  # right to left; LIFO
+                    zigzag_node_vals[level].appendleft(node.val)
+            nodes = next_level_nodes
+            next_level_nodes = []
+            level += 1
+            zigzag_node_vals.append(deque([]))
+        return [list(deq) for deq in zigzag_node_vals]
+```
+- 点を走査する順番自体を工夫するのか、あるいは値の追加の仕方を工夫するのか。
+    - 今回なら、点は普通に左から順に走査していき、値の追加の仕方をzigzagにするのが楽
+- 初期値の設定、各ループでの更新のタイミングは統一するのが良い
+    - 例えば、`next_level_nodes`の初期化をループの外で行い、ループの最後で空にするようにした
+    - `level`の初期化をループの外で行い、ループの最後でインクリメントするようにした
+
+
+
+`nodes`をキューで管理して、ネストを1つ浅くする方法（`step3_2.py`）
+```python
+from collections import deque
+
+
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        zigzag_node_vals: list[deque[int]] = []  # deque
+        nodes_queue = deque([(root, 0)])
+        while nodes_queue:
+            node, level = nodes_queue.popleft()
+            if node is None:
+                continue
+            while len(zigzag_node_vals) <= level:
+                zigzag_node_vals.append(deque([]))
+            if level % 2 == 0:  # from left to right; FIFO
+                zigzag_node_vals[level].append(node.val)
+            else:  # from right to left; LIFO
+                zigzag_node_vals[level].appendleft(node.val)
+
+            nodes_queue.append((node.left, level + 1))
+            nodes_queue.append((node.right, level + 1))
+        return [list(deq) for deq in zigzag_node_vals]
+```
+
+
+`reverse`を使う方法（`step3_3.py`）
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        zigzag_level_order_vals: list[list[int]] = []  # deque
+        nodes = [root]
+        next_level_nodes = []
+        level = 0
+
+        while True:
+            zigzag_vals = []
+            for node in nodes:
+                if node is None:
+                    continue
+                zigzag_vals.append(node.val)
+                next_level_nodes.append(node.left)
+                next_level_nodes.append(node.right)
+
+            if not next_level_nodes:
+                return zigzag_level_order_vals
+            # align node vals from right to left
+            if level % 2 == 1:
+                zigzag_vals.reverse()
+            zigzag_level_order_vals.append(zigzag_vals)
+            nodes = next_level_nodes
+            next_level_nodes = []
+            level += 1
+```
+- `deque`を使わずに、普通のリストで値を保持し、`reverse`で並び替える方法
+- levelごとにループを回すので、ネストは深くなるが、どのコードよりもzigzagの処理ががシンプルになる
+
+
+## step4 (FB)
+
+
+
+# 別解・模範解答
+## DFS
+
+```python
+from collections import deque
+
+
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        if not root:
+            return []
+        nodes_by_level = []
+
+        def traverse(node: TreeNode, level: int):
+            while len(nodes_by_level) <= level:
+                nodes_by_level.append(deque([]))
+            if level % 2 == 0:
+                nodes_by_level[level].append(node.val)
+            else:
+                nodes_by_level[level].appendleft(node.val)
+
+            if node.left:
+                traverse(node.left, level + 1)
+            if node.right:
+                traverse(node.right, level + 1)
+
+        traverse(root, 0)
+
+        return [list(d) for d in nodes_by_level]
+```
+
+- 再帰関数を用いてグラフを走査していく。
+- 各レベルごとのノードのリストを`deque`として保持して、
+    - `level` が奇数のときは走査した順、つまりFIFOの順にノードを追加していく。
+    - `level` が偶数のときは逆順、つまりLIFOの順にノードを追加していく。
+- 最後に`deque`をリストに変換して返す。
+
+
+- 時間計算量：`O(n)`
+- 空間計算量：`O(n)`

103_binary_tree_zigzag_level_order_traversal_medium/dfs.py

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+#
+# @lc app=leetcode id=103 lang=python3
+#
+# [103] Binary Tree Zigzag Level Order Traversal
+#
+
+# @lc code=start
+# 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
+
+
+from collections import deque
+
+
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        if not root:
+            return []
+        nodes_by_level = []
+
+        def traverse(node: TreeNode, level: int):
+            while len(nodes_by_level) <= level:
+                nodes_by_level.append(deque([]))
+            if level % 2 == 0:
+                nodes_by_level[level].append(node.val)
+            else:
+                nodes_by_level[level].appendleft(node.val)
+
+            if node.left:
+                traverse(node.left, level + 1)
+            if node.right:
+                traverse(node.right, level + 1)
+
+        traverse(root, 0)
+
+        return [list(d) for d in nodes_by_level]
+
+
+# @lc code=end

103_binary_tree_zigzag_level_order_traversal_medium/step1.py

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+#
+# @lc app=leetcode id=103 lang=python3
+#
+# [103] Binary Tree Zigzag Level Order Traversal
+#
+
+# @lc code=start
+# 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]]:
+        if not root:
+            return []
+        LEFT_TO_RIGHT = 1
+        RIGHT_TO_LEFT = -1
+        direction = LEFT_TO_RIGHT
+        nodes = [root]
+        zigzag_level_nodes = []
+        while nodes:
+            node_vals = []
+            next_level_nodes = []
+            while nodes:
+                node = nodes.pop()
+                node_vals.append(node.val)
+                if direction == LEFT_TO_RIGHT:
+                    if node.left:
+                        next_level_nodes.append(node.left)
+                    if node.right:
+                        next_level_nodes.append(node.right)
+                else:
+                    if node.right:
+                        next_level_nodes.append(node.right)
+                    if node.left:
+                        next_level_nodes.append(node.left)
+
+            zigzag_level_nodes.append(node_vals)
+            nodes = next_level_nodes
+            direction *= -1
+        return zigzag_level_nodes
+
+
+# @lc code=end

103_binary_tree_zigzag_level_order_traversal_medium/step2.py

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+#
+# @lc app=leetcode id=103 lang=python3
+#
+# [103] Binary Tree Zigzag Level Order Traversal
+#
+
+# @lc code=start
+# 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
+
+
+from collections import deque
+
+
+class Solution:
+    def zigzagLevelOrder(self, root: TreeNode | None) -> list[list[int]]:
+        if not root:
+            return []
+        zigzag_level_nodes = []
+        nodes = deque([(root, 0)])
+        while nodes:
+            next_nodes = deque([])
+            while nodes:
+                node, level = nodes.popleft()
+                while len(zigzag_level_nodes) <= level:
+                    zigzag_level_nodes.append(deque([]))
+
+                # left to right
+                if level % 2 == 0:
+                    zigzag_level_nodes[level].append(node.val)
+                # right to left
+                else:
+                    zigzag_level_nodes[level].appendleft(node.val)
+
+                if node.left:
+                    next_nodes.append((node.left, level + 1))
+                if node.right:
+                    next_nodes.append((node.right, level + 1))
+
+            nodes = next_nodes
+
+        return [list(deq) for deq in zigzag_level_nodes]
+
+
+# @lc code=end