206 reverse linked list

memo.md

@@ -1 +1,125 @@
 # Step1
+
+## アプローチ
+
+* 数字を順番に見ていってリストに内容を保存. リストの後ろを見ながら新しいノードを作成していく
+* 前から順番に繋ぎ変えていく.
+    * in-placeでも, 別のノードリストを用意してもできる.
+* 再帰で実装もできるけど本質的にはやること一緒.
+
+## Code1-1 (別のリストに数字を保存)
+
+```python
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return None
+        values = []
+        while head is not None:
+            values.append(head.val)
+            head = head.next
+        reversed_head_dummy = ListNode()
+        reversed_tail = reversed_head_dummy
+        for i in range(len(values) - 1, -1, -1):
+            node = ListNode(values[i])
+            reversed_tail.next = node
+            reversed_tail = reversed_tail.next
+        return reversed_head_dummy.next
+```
+
+## Code1-2 (別のメモリ使用. one path)
+
+```python
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return None
+        reversed_head = None
+        node = head
+        while node is not None:
+            new_node = ListNode(node.val)
+            new_node.next = reversed_head
+            reversed_head = new_node
+            node = node.next
+        return reversed_head
+```
+
+## Code1-3 (in-place, one path)
+
+```python
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return None
+        reversed_head = None
+        node = head
+        while node is not None:
+            next_node = node.next
+            node.next = reversed_head
+            reversed_head = node
+            node = next_node
+        return reversed_head
+```
+
+# Step2
+
+変更なし
+
+# Step3
+
+## Code1-1 (別のリストに数字を保存)
+
+* `values[::-1]`とするよりも, `range(len(values) - 1, -1, -1)`を使った方が, 配列が新たにコピーされなくて済むので効率的かと思ったが, 結局`range()`も新しく配列を新たに作成する上, 中に含まれるのはどちらも`int`型だから変わらない気がしてきた.
+
+```python
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return None
+        values = []
+        node = head
+        while node is not None:
+            values.append(node.val)
+            node = node.next
+        reversed_dummy_head = ListNode()
+        reversed_tail = reversed_dummy_head
+        for i in range(len(values) - 1, -1, -1):
+            new_node = ListNode(values[i])
+            reversed_tail.next = new_node
+            reversed_tail = reversed_tail.next
+        return reversed_dummy_head.next
+```
+
+## Code1-2 (別のメモリ使用. one path)
+
+```python
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return None
+        node = head
+        reversed_head = None
+        while node is not None:
+            new_node = ListNode(node.val)
+            new_node.next = reversed_head
+            reversed_head = new_node
+            node = node.next
+        return reversed_head
+```
+
+## Code1-3 (in-place, one path)
+
+```python
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None:
+            return None
+        prev = None
+        node = head
+        while node is not None:
+            next_node = node.next
+            node.next = prev
+            prev = node
+            node = next_node
+        return prev
+```