diff --git a/memo.md b/memo.md
index 4bd0397..7db2646 100644
--- a/memo.md
+++ b/memo.md
@@ -1 +1,187 @@
 # Step1
+
+## アプローチ (14:22)
+
+* rootとなる値がわかれば, それより左側と右側で再帰を呼ぶことで解ける
+    * rootとなる値は中央値?
+    * height-balancedではある
+    * rootの左側と右側の個数が等しくなるようにすればbalancedにはなるよな
+* 日本語で仕事を考えてみる
+    * 与えられた数字の列からrootになる値を決める(自分の仕事)
+    * 決めた数字の左側と右側の配列を部下に渡して, 自分が繋ぐべきrootをもらう
+    * rootを返す
+* 計算量を考える
+    * 計算量をT(N)とする
+    * T(N) = T(N/2) + T(N/2) = T(N / 2^2) + ... + T(N / 2^2)
+    * T(N) = T(1) * 2 * 2^(log2_N)
+    * O(N)
+        * 追記: slicingを使っていた場合はO(NlogN)になる
+* 空間計算量
+    * 再帰の最大回数 ... logN
+    * 結果として使う分 ... size(TreeNode) * N
+    * O(N)
+
+## Code1-1 (Recursion) - 1:49
+
+```python
+from typing import List, Optional
+
+
+# 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
+        if len(nums) == 1:
+            return TreeNode(nums[0])
+
+        mid_idx = len(nums) // 2
+        root_node = TreeNode(nums[mid_idx])
+        left_nums = nums[:mid_idx]
+        right_nums = nums[mid_idx + 1:]
+        root_node.left = self.sortedArrayToBST(left_nums)
+        root_node.right = self.sortedArrayToBST(right_nums)
+        return root_node
+
+```
+
+# Step2
+
+## 他の人のコード
+
+* https://github.com/mamo3gr/arai60/pull/23/files
+    * whileを使用した再帰
+    * 区間を指定した再帰関数を作った実装もある
+
+## Code2-1 (Recursion)
+
+```python
+from typing import List, Optional
+
+
+# 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 sortedArrayToBST_with_range(nums, left, right):
+            if left > right:
+                return None
+            if left == right:
+                return TreeNode(nums[left])
+            
+            mid_idx = (left + right) // 2
+            return TreeNode(
+                    val=nums[mid_idx], 
+                    left=sortedArrayToBST_with_range(nums, left, mid_idx - 1),
+                    right=sortedArrayToBST_with_range(nums, mid_idx + 1, right)
+                )
+            
+        return sortedArrayToBST_with_range(nums, 0, len(nums) - 1)
+
+
+```
+
+## Code2-2 (DFS)
+
+```python
+from typing import List, Optional
+
+
+# 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]:
+        root = TreeNode()
+        frontier = [(0, len(nums) - 1, root)]
+        while frontier:
+            left, right, node = frontier.pop()
+            mid = (left + right) // 2
+            node.val = nums[mid]
+            if left <= mid - 1:
+                node.left = TreeNode()
+                frontier.append((left, mid - 1, node.left))
+            if mid + 1 <= right:
+                node.right = TreeNode()
+                frontier.append((mid + 1, right, node.right))
+        return root
+
+```
+
+# Step3
+
+## Code3-1 (Recursion)
+
+```python
+# 1st: 1:41
+# 1st: 1:19
+# 1st: 1:12
+
+
+# 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]) -> TreeNode | None:
+        def sortedArrayToBST_with_range(nums, left, right):
+            if left > right:
+                return None
+            if left == right:
+                return TreeNode(nums[left])
+            
+            mid = (left + right) // 2
+            return TreeNode(
+                val=nums[mid],
+                left=sortedArrayToBST_with_range(nums, left, mid - 1),
+                right=sortedArrayToBST_with_range(nums, mid + 1, right)
+            )
+        return sortedArrayToBST_with_range(nums, 0, len(nums) - 1)
+
+```
+
+## Code3-2 (DFS)
+
+```python
+# 1st: 1:55
+# 1st: 1:41
+# 1st: 1:25
+
+# 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]) -> TreeNode | None:
+        root = TreeNode()
+        frontier = [(0, len(nums) - 1, root)]
+        while frontier:
+            left, right, node = frontier.pop()
+            mid = (left + right) // 2
+            node.val = nums[mid]
+            if left <= mid - 1:
+                node.left = TreeNode()
+                frontier.append((left, mid - 1, node.left))
+            if mid + 1 <= right:
+                node.right = TreeNode()
+                frontier.append((mid + 1, right, node.right))
+            
+        return root
+
+```
\ No newline at end of file
diff --git a/step1-1_recursion.py b/step1-1_recursion.py
new file mode 100644
index 0000000..fe47b40
--- /dev/null
+++ b/step1-1_recursion.py
@@ -0,0 +1,23 @@
+from typing import List, Optional
+
+
+# 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
+        if len(nums) == 1:
+            return TreeNode(nums[0])
+
+        mid_idx = len(nums) // 2
+        root_node = TreeNode(nums[mid_idx])
+        left_nums = nums[:mid_idx]
+        right_nums = nums[mid_idx + 1:]
+        root_node.left = self.sortedArrayToBST(left_nums)
+        root_node.right = self.sortedArrayToBST(right_nums)
+        return root_node
\ No newline at end of file
diff --git a/step2-1_recursion.py b/step2-1_recursion.py
new file mode 100644
index 0000000..45ce03d
--- /dev/null
+++ b/step2-1_recursion.py
@@ -0,0 +1,25 @@
+from typing import List, Optional
+
+
+# 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 sortedArrayToBST_with_range(nums, left, right):
+            if left > right:
+                return None
+            if left == right:
+                return TreeNode(nums[left])
+            
+            mid_idx = (left + right) // 2
+            return TreeNode(
+                    val=nums[mid_idx], 
+                    left=sortedArrayToBST_with_range(nums, left, mid_idx - 1),
+                    right=sortedArrayToBST_with_range(nums, mid_idx + 1, right)
+                )
+            
+        return sortedArrayToBST_with_range(nums, 0, len(nums) - 1)
diff --git a/step2-2_dfs.py b/step2-2_dfs.py
new file mode 100644
index 0000000..e395735
--- /dev/null
+++ b/step2-2_dfs.py
@@ -0,0 +1,24 @@
+from typing import List, Optional
+
+
+# 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]:
+        root = TreeNode()
+        frontier = [(0, len(nums) - 1, root)]
+        while frontier:
+            left, right, node = frontier.pop()
+            mid = (left + right) // 2
+            node.val = nums[mid]
+            if left <= mid - 1:
+                node.left = TreeNode()
+                frontier.append((left, mid - 1, node.left))
+            if mid + 1 <= right:
+                node.right = TreeNode()
+                frontier.append((mid + 1, right, node.right))
+        return root
\ No newline at end of file
diff --git a/step3-1_recursion.py b/step3-1_recursion.py
new file mode 100644
index 0000000..af25814
--- /dev/null
+++ b/step3-1_recursion.py
@@ -0,0 +1,26 @@
+# 1st: 1:41
+# 1st: 1:19
+# 1st: 1:12
+
+
+# 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]) -> TreeNode | None:
+        def sortedArrayToBST_with_range(nums, left, right):
+            if left > right:
+                return None
+            if left == right:
+                return TreeNode(nums[left])
+            
+            mid = (left + right) // 2
+            return TreeNode(
+                val=nums[mid],
+                left=sortedArrayToBST_with_range(nums, left, mid - 1),
+                right=sortedArrayToBST_with_range(nums, mid + 1, right)
+            )
+        return sortedArrayToBST_with_range(nums, 0, len(nums) - 1)
\ No newline at end of file
diff --git a/step3-2_dfs.py b/step3-2_dfs.py
new file mode 100644
index 0000000..43513ad
--- /dev/null
+++ b/step3-2_dfs.py
@@ -0,0 +1,26 @@
+# 1st: 1:55
+# 1st: 1:41
+# 1st: 1:25
+
+# 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]) -> TreeNode | None:
+        root = TreeNode()
+        frontier = [(0, len(nums) - 1, root)]
+        while frontier:
+            left, right, node = frontier.pop()
+            mid = (left + right) // 2
+            node.val = nums[mid]
+            if left <= mid - 1:
+                node.left = TreeNode()
+                frontier.append((left, mid - 1, node.left))
+            if mid + 1 <= right:
+                node.right = TreeNode()
+                frontier.append((mid + 1, right, node.right))
+            
+        return root
\ No newline at end of file