From ae37ad0f8feaad7a81cb22ab4937aaca538e30dd Mon Sep 17 00:00:00 2001
From: Hiro Funatsuka <hiro.funatsuka@gmail.com>
Date: Sun, 26 Apr 2026 22:56:44 +0900
Subject: [PATCH] Solve convert sorted array to binary tree

---
 README.md |  2 +-
 step1.md  | 29 +++++++++++++++++++++++++++--
 step2.md  | 17 +++++++++++++++++
 step3.md  | 24 +++++++++++++++++++++---
 4 files changed, 66 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 0ee68b0..d245ac4 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # LeetCode
 
-## 問題: [Link Text](URL)
+## 問題: [108. Convert Sorted Array to Binary Tree](https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/description/)
 
 ## 前提
 
diff --git a/step1.md b/step1.md
index 54ebbc1..7599776 100644
--- a/step1.md
+++ b/step1.md
@@ -1,7 +1,32 @@
 # Step 1
 
+1. numsが空ならNoneを返す
+2. rootはnumsの真ん中のindexの値
+3. rootのleftは2のindexまでのnumsで再度2を繰り返す
+4. rootのrightは2のindexより先のnumsで再度2を繰り返す
+
+- 配列のスライスの仕方を正確に覚えていない
+
 ```python
+# 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_index = (len(nums) - 1) // 2
+        root = TreeNode(nums[middle_index])
+        root.left = self.sortedArrayToBST(nums[:middle_index])
+        root.right = self.sortedArrayToBST(nums[middle_index + 1:])
+        return root
 ```
-時間計算量:
 
-空間計算量:
+時間計算量: O(nlogn)
+
+空間計算量: O(n)
+
+- スライス使っているのが非効率そう
diff --git a/step2.md b/step2.md
index 6ec3360..ecbde55 100644
--- a/step2.md
+++ b/step2.md
@@ -1,4 +1,21 @@
 # Step 2
 
 ```python
+# 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 helper(l, r):
+            if l > r:
+                return None
+            middle_index = (l + r) // 2
+            root = TreeNode(nums[middle_index])
+            root.left = helper(l, middle_index - 1)
+            root.right = helper(middle_index + 1, r)
+            return root
+        return helper(0, len(nums) - 1)
 ```
diff --git a/step3.md b/step3.md
index 522daa6..ec4591a 100644
--- a/step3.md
+++ b/step3.md
@@ -1,9 +1,27 @@
 # Step 3
 
 ```python
+# 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 helper(l, r):
+            if l > r:
+                return None
+            middle_index = (l + r) // 2
+            root = TreeNode(nums[middle_index])
+            root.left = helper(l, middle_index - 1)
+            root.right = helper(middle_index + 1, r)
+            return root
+        return helper(0, len(nums) - 1)
 ```
-1回目: 分　秒
 
-2回目: 分　秒
+1回目: 1分　54秒
 
-3回目: 分　秒
+2回目: 1分　55秒
+
+3回目: 1分　43秒