diff --git a/108-Convert-Sorted-Array-to-Binary-Search-Tree/note.md b/108-Convert-Sorted-Array-to-Binary-Search-Tree/note.md
new file mode 100644
index 0000000..4367366
--- /dev/null
+++ b/108-Convert-Sorted-Array-to-Binary-Search-Tree/note.md
@@ -0,0 +1,40 @@
+# 108. Convert Sorted Array to Binary Search Tree
+
+https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/description/
+
+## step1（まず通す）
+
+元の配列がソートされているので、二分探索する要領で左右配列の分かれ目（中間）を繋いでいけばいい
+再帰で書くのがわかりやすい。
+
+## step2（整形＆他の人のコードを読む）
+
+ループでも書く。部分配列を毎回スタックに詰めるのは効率が悪いので、区間の先頭と末尾のインデックスだけ持つようにする。step1.py は毎回配列をスライスしているのでちょっと遅い。スライスする長さは木の階層ごとに半分ずつになっていくのでn := len(nums) としたとき時間計算量は O(n log n)。
+
+- https://github.com/naoto-iwase/leetcode/pulls
+  - 「（再帰の base case は）1も含めると直感で倍速くなる。(二分木の構造または本問の再帰構造より）」
+    - 確かにそうだ。葉の左右の子（None）に対する関数呼び出しがなくなるので。
+- https://github.com/shintaro1993/arai60/pull/28/changes#diff-5db3d349f04f651e2faacefca7e2937a523f2fd8c8b6cbd30ad1e7ed699f8d3aR36
+  - 添え字の再帰
+
+```python
+class Solution:
+    def sortedArrayToBST(self, nums: list[int]) -> Optional[TreeNode]:
+        def interval_to_BST(head, tail) -> Optional[TreeNode]:
+            if head == tail:
+                return None
+
+            center = (tail + head) // 2
+            return TreeNode(
+                val=nums[center],
+                left=interval_to_BST(head, center),
+                right=interval_to_BST(center + 1, tail),
+            )
+
+        return interval_to_BST(0, len(nums))
+```
+
+- https://github.com/shintaro1993/arai60/pull/28/changes#diff-5db3d349f04f651e2faacefca7e2937a523f2fd8c8b6cbd30ad1e7ed699f8d3aR91
+  - 添え字の範囲について。「自分は、0 と len(nums) で始めるのが好きだと思った。」私も同じ好み。
+
+## step3（10分以内にさっとかける \* 3回）
diff --git a/108-Convert-Sorted-Array-to-Binary-Search-Tree/step1.py b/108-Convert-Sorted-Array-to-Binary-Search-Tree/step1.py
new file mode 100644
index 0000000..5084aa2
--- /dev/null
+++ b/108-Convert-Sorted-Array-to-Binary-Search-Tree/step1.py
@@ -0,0 +1,21 @@
+from typing import Optional
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def sortedArrayToBST(self, nums: list[int]) -> Optional[TreeNode]:
+        if not nums:
+            return None
+
+        center = len(nums) // 2
+        return TreeNode(
+            val=nums[center],
+            left=self.sortedArrayToBST(nums[:center]),
+            right=self.sortedArrayToBST(nums[center + 1 :]),
+        )
diff --git a/108-Convert-Sorted-Array-to-Binary-Search-Tree/step2.py b/108-Convert-Sorted-Array-to-Binary-Search-Tree/step2.py
new file mode 100644
index 0000000..e69dc94
--- /dev/null
+++ b/108-Convert-Sorted-Array-to-Binary-Search-Tree/step2.py
@@ -0,0 +1,29 @@
+from typing import Optional
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def sortedArrayToBST(self, nums: list[int]) -> Optional[TreeNode]:
+        if not nums:
+            return None
+
+        root = TreeNode()
+        stack: list[tuple[int, int, TreeNode]] = [(0, len(nums), root)]
+        while stack:
+            head, tail, node = stack.pop()
+            center = (tail + head) // 2
+            node.val = nums[center]
+            if head < center:
+                node.left = TreeNode()
+                stack.append((head, center, node.left))
+            if center + 1 < tail:
+                node.right = TreeNode()
+                stack.append((center + 1, tail, node.right))
+
+        return root
diff --git a/108-Convert-Sorted-Array-to-Binary-Search-Tree/step3.py b/108-Convert-Sorted-Array-to-Binary-Search-Tree/step3.py
new file mode 100644
index 0000000..b72278a
--- /dev/null
+++ b/108-Convert-Sorted-Array-to-Binary-Search-Tree/step3.py
@@ -0,0 +1,26 @@
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def sortedArrayToBST(self, nums: list[int]) -> Optional[TreeNode]:
+        if not nums:
+            return None
+
+        root = TreeNode()
+        stack: list[tuple[int, int, TreeNode]] = [(0, len(nums), root)]
+        while stack:
+            head, tail, node = stack.pop()
+            center = (head + tail) // 2
+            node.val = nums[center]
+            if head < center:
+                node.left = TreeNode()
+                stack.append((head, center, node.left))
+            if center + 1 < tail:
+                node.right = TreeNode()
+                stack.append((center + 1, tail, node.right))
+
+        return root