diff --git a/108-convert-sorted-array-to-binary-search-tree/memo.md b/108-convert-sorted-array-to-binary-search-tree/memo.md
new file mode 100644
index 0000000..71991c1
--- /dev/null
+++ b/108-convert-sorted-array-to-binary-search-tree/memo.md
@@ -0,0 +1,10 @@
+## step1で考えたこと
+root nodeをどう決めるか？ => numsの中央値を取る？
+
+Binary Searchが使えるわけではなさそう？
+
+middleが中心のIndexとなるからそれの一個ずらした左右のIndexをleft, rightとして、端に寄せていくイメージ
+
+## step1に対するフィードバック
+- bst.left = nums[left] / bst.right = nums[right] って int を入れてしまっていて、TreeNode をつないでない（木になってない）
+- そもそもどんどん左に、右にnodeを増やしていったらそれはHeight-Balancedではないのでは？
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..adf1341
--- /dev/null
+++ b/108-convert-sorted-array-to-binary-search-tree/step1.py
@@ -0,0 +1,31 @@
+# step1で書いたコード（動かないコード）
+
+# 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 sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
+        middle = len(nums) // 2
+        bst = TreeNode(nums[middle])
+        # middleの両隣のindexをleft, rightとする
+        left = middle - 1
+        right = middle + 1
+
+        while left > 0 or right < len(nums) - 1:
+            if left < 0:
+                break
+            else:
+                bst.left = nums[left]
+                left -= 1
+
+            if right > len(nums) - 1:
+                break
+            else:
+                bst.right = nums[right]
+                right += 1
+        
+        return bst
+        
\ No newline at end of file
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..c435df7
--- /dev/null
+++ b/108-convert-sorted-array-to-binary-search-tree/step2.py
@@ -0,0 +1,22 @@
+# step1の書き方に一番近い、正解のコード
+# 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 sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
+        if not nums:
+            return None
+
+        middle = len(nums) // 2
+        bst = TreeNode(nums[middle])
+        
+        left_nums = nums[:middle]
+        right_nums = nums[middle + 1:]
+
+        bst.left = self.sortedArrayToBST(left_nums)
+        bst.right = self.sortedArrayToBST(right_nums)
+        
+        return bst
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..b0f2d18
--- /dev/null
+++ b/108-convert-sorted-array-to-binary-search-tree/step3.py
@@ -0,0 +1,20 @@
+# 最適解のコード
+
+# 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 sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
+        def buildBST(left: int, right: int) -> Optional[TreeNode]:
+            if left > right:
+                return None
+            
+            middle = (left + right) // 2
+            root = TreeNode(nums[middle])
+            root.left = buildBST(left, middle - 1)
+            root.right = buildBST(middle + 1, right)
+            return root
+        return buildBST(0, len(nums) - 1)
\ No newline at end of file