Create 103. Binary Tree Zigzag Level Order Traversal.md

競技プロ就活部PR用/103. Binary Tree Zigzag Level Order Traversal.md

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+## DFSによる解答
+### 1回目 (29m25s)
+かなり悩んだ。どの順番で何をすべきかを整理するのに時間がかかった。
+→BFSを検討するべき。
+
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return []
+
+        reverse = 1
+        zigzag_level_ordered_nodes = []
+        next_nodes = deque([(root)])
+
+        while next_nodes:
+            level_nodes = []
+            for _ in range(len(next_nodes)):
+                if reverse == 1:
+                    node = next_nodes.popleft()
+                    if node.left: next_nodes.append(node.left)
+                    if node.right: next_nodes.append(node.right)
+
+                if reverse == -1:
+                    node = next_nodes.pop()
+                    if node.right: next_nodes.appendleft(node.right)
+                    if node.left: next_nodes.appendleft(node.left)
+
+
+                level_nodes.append(node.val)
+
+            reverse *= -1
+            zigzag_level_ordered_nodes.append(level_nodes)
+
+        return zigzag_level_ordered_nodes
+```
+
+上記のコードを基にセルフ修正。
+命名と、reverseに意味がない(なにがreverseなのかわからない)ため、
+階数(level)を数え上げていく方法に変更。
+levelは1始まりにしても良いかもしれない。
+
+### 2回目
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return []
+
+        level = 0
+        zigzag_ordered_level_nodes = []
+        next_level_nodes = deque([root])
+
+        while next_level_nodes:
+            level_nodes = []
+            for _ in range(len(next_level_nodes)):
+                if level % 2 == 0:
+                    node = next_level_nodes.popleft()
+                    if node.left: next_level_nodes.append(node.left)
+                    if node.right: next_level_nodes.append(node.right)
+                else:
+                    node = next_level_nodes.pop()
+                    if node.right: next_level_nodes.appendleft(node.right)
+                    if node.left: next_level_nodes.appendleft(node.left)
+
+                level_nodes.append(node.val)
+            level += 1
+            zigzag_ordered_level_nodes.append(level_nodes)
+
+        return zigzag_ordered_level_nodes
+```
+
+コメントを基に、level_nodes_valuesに変更。少し変数名が長く(ややこしく)なりすぎているかもしれない。
+特にzigzag_ordered_level_nodes。
+### 3回目
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return []
+        level = 0
+        zigzag_ordered_level_nodes = []
+        next_level_nodes = deque([root])
+
+        while next_level_nodes:
+            level_nodes_values = []
+            for _ in range(len(next_level_nodes)):
+                if level % 2 == 0:
+                    node = next_level_nodes.popleft()
+                    if node.left: next_level_nodes.append(node.left)
+                    if node.right: next_level_nodes.append(node.right)
+                else:
+                    node = next_level_nodes.pop()
+                    if node.right: next_level_nodes.appendleft(node.right)
+                    if node.left: next_level_nodes.appendleft(node.left)
+
+                level_nodes_values.append(node.val)
+            level += 1
+            zigzag_ordered_level_nodes.append(level_nodes_values)
+
+        return zigzag_ordered_level_nodes
+```
+
+
+BFSの方が理解しやすかった。
+## BFSによる解答
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return []
+        next_level_nodes = deque([root])
+        level = 0
+        zigzag_ordered_level_nodes = []
+
+        while next_level_nodes:
+            level_node_values = []            
+            for _ in range(len(next_level_nodes)):
+                node = next_level_nodes.popleft()
+                if node.left: next_level_nodes.append(node.left)
+                if node.right: next_level_nodes.append(node.right)
+                level_node_values.append(node.val)
+
+            if level % 2:
+                level_node_values.reverse()
+            level += 1
+            zigzag_ordered_level_nodes.append(level_node_values)
+        return zigzag_ordered_level_nodes
+```
+
+
+### 4回目
+ahayashiさんのアドバイスを基に書き直し。level_orered_valsへの値の追加以外の処理を共通化。
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root: return []
+        zigzag_ordered_level_nodes = []
+        next_level_nodes = deque([(root)])
+        level = 1
+
+        while next_level_nodes:
+            level_ordered_vals = deque()
+            for _ in range(len(next_level_nodes)):
+                node = next_level_nodes.popleft()
+
+                if level % 2:
+                    level_ordered_vals.append(node.val)
+                else:
+                    level_ordered_vals.appendleft(node.val)
+
+                if node.left: next_level_nodes.append(node.left)
+                if node.right: next_level_nodes.append(node.right)
+            zigzag_ordered_level_nodes.append(level_ordered_vals)
+            level += 1
+
+        return zigzag_ordered_level_nodes
+```
+
+補助関数を用いた方法 (levelの値に基づいて最終結果の対応indexに値を加えていく, reverseなし)
+```python
+class Solution:
+    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        # DFS
+        if not root: return []
+        zigzag_ordered_level_nodes = [deque()]
+
+        def zigzagLevelOrder_helper(node, level): #Note:level (int) counts from 0.
+            while len(zigzag_ordered_level_nodes) <= level:
+                    zigzag_ordered_level_nodes.append(deque())
+            if level % 2:
+                zigzag_ordered_level_nodes[level].appendleft(node.val)
+            else:
+                zigzag_ordered_level_nodes[level].append(node.val)
+
+            if node.left: zigzagLevelOrder_helper(node.left, level + 1)
+            if node.right: zigzagLevelOrder_helper(node.right, level + 1)
+
+        zigzagLevelOrder_helper(root, 0)
+        return zigzag_ordered_level_nodes
+```
+
+
+
+