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+
+## 再帰による解答
+---
+### 1~3回目
+12m25s<br>
+preorder, inorderについて考えていたためか、整理できた。
+本番でこの問題を緊張感の中で解けるかはあまり自信がない。
+変数が少なく、2~3回目において大きな変更はなし。
+時間計算量: O(N)<br>
+空間計算量: O(N)<br>
+
+```python
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if not preorder or not inorder:
+            return None
+        root = TreeNode(preorder[0])
+        index_in_inorder = inorder.index(preorder[0])
+        root.left = self.buildTree(preorder[1:index_in_inorder+1], inorder[:index_in_inorder])
+        root.right = self.buildTree(preorder[index_in_inorder+1:], inorder[index_in_inorder+1:])
+
+        return root
+```
+
+### stackを利用した解法
+```python
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if not preorder or not inorder:
+            return None
+
+        root = TreeNode(preorder[0])
+        nodes_with_index = [(root, 0, len(inorder) - 1, 0, len(preorder) - 1)]
+        inorder_index_map = {value: index for index, value in enumerate(inorder)}
+
+        while nodes_with_index:
+            node, in_start, in_end, pre_start, pre_end = nodes_with_index.pop()
+            if pre_start > pre_end or in_start > in_end:
+                continue
+
+            root_val = preorder[pre_start]
+            mid = inorder_index_map[root_val]
+            left_count = mid - in_start
+
+            if left_count > 0:
+                left_node = TreeNode(preorder[pre_start + 1])
+                node.left = left_node
+                nodes_with_index.append((left_node, in_start, mid - 1, pre_start + 1, pre_start + left_count))
+
+            if mid < in_end:
+                right_node = TreeNode(preorder[pre_start + left_count + 1])
+                node.right = right_node
+                nodes_with_index.append((right_node, mid + 1, in_end, pre_start + left_count + 1, pre_end))
+
+        return root
+```
+
+right_countを定義して利用
+変数名: root_val → node_val
+```python
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if not preorder or not inorder:
+            return None
+
+        root = TreeNode(preorder[0])
+        nodes_with_index = [(root, 0, len(inorder) - 1, 0, len(preorder) - 1)]
+        inorder_index_map = {value: index for index, value in enumerate(inorder)}
+
+        while nodes_with_index:
+            node, in_start, in_end, pre_start, pre_end = nodes_with_index.pop()
+            if pre_start > pre_end or in_start > in_end:
+                continue
+
+            node_val = preorder[pre_start]
+            mid = inorder_index_map[node_val]
+            left_count = mid - in_start
+            right_count = in_end - mid
+
+            if left_count > 0:
+                left_node = TreeNode(preorder[pre_start + 1])
+                node.left = left_node
+                nodes_with_index.append((left_node, in_start, mid - 1, pre_start + 1, pre_start + left_count))
+
+            if right_count > 0:
+                right_node = TreeNode(preorder[pre_start + left_count + 1])
+                node.right = right_node
+                nodes_with_index.append((right_node, mid + 1, in_end, pre_start + left_count + 1, pre_end))
+
+        return root
+```
+
+## HashMapを用いた解法 
+---
+ahayashiさん、kandaさんの解答を参考に、HashMapと関数化を利用した解答も書きました。
+```python
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        def buildtree_helper(left_bound, right_bound):
+            nonlocal preorder_index
+            if left_bound >= right_bound:
+                return
+            
+            root_value = preorder[preorder_index]
+            root = TreeNode(root_value)
+
+            inorder_index = inorder_value_to_index[root_value]
+            preorder_index += 1
+            root.left = buildtree_helper(left_bound, inorder_index)
+            root.right = buildtree_helper(inorder_index + 1, right_bound) 
+            return root
+
+        preorder_index = 0
+        inorder_value_to_index = {}
+        for index, value in enumerate(inorder):
+            inorder_value_to_index[value] = index
+
+        return buildtree_helper(0, len(inorder))
+```
+