82. Remove Duplicates from Sorted List II

82. Remove Duplicates from Sorted List II/memo.md

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+
+## step1:
+まず順番にノードを探索していって、head.valがhead.next.valと一致していればそれは重複したノードなのでそれらが消えるまでheadを進めていく。もしhead.valがhead.next.valでなければそれはdistinctなので答えで返すnodeとして追加するという方針で行く。しかし返すべき値をresとしてどのようにres自身に代入するか迷ったので、get_distinct_listという帰納関数を作りボトムアップでdistinctな連結リストを得て返すという方法を取った。
+
+### code
+```python
+class Solution:
+    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
+
+        def get_distinct_list(head_arg):
+            if not head_arg or not head_arg.next:
+                return head_arg
+            if head_arg.val == head_arg.next.val:
+                while head_arg.next and head_arg.val == head_arg.next.val:
+                    head_arg = head_arg.next
+                head_arg = head_arg.next
+                return get_distinct_list(head_arg)
+
+            head_arg.next = get_distinct_list(head_arg.next)
+            return head_arg
+
+        return get_distinct_list(head)
+```
+
+## step2:
+get_distinct_listがやっていることとdeleteDuplicatesがやってることが同じだと気づいたのでdeleteDuplicates内でのget_distinct_listの宣言をやめてdeleteDuplicatesの帰納的呼び出しを行うことにした。
+
+### code
+```python
+class Solution:
+    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
+
+        if not head or not head.next:
+            return head
+
+        if head.val == head.next.val:
+            while head.next and head.val == head.next.val:
+                head = head.next
+            head = head.next
+            return self.deleteDuplicates(head)
+
+        head.next = self.deleteDuplicates(head.next)
+        return head
+
+```
+
+
+## step3:
+duplicateだった場合に最後head = head.nextとやってるがそのままhead.nextを代入すればいいと気づいたのでhead = head.nextを消した。
+
+### code
+```python
+class Solution:
+    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head or not head.next:
+            return head
+
+        if head.val == head.next.val:
+            while head.next and head.val == head.next.val:
+                head = head.next
+            return self.deleteDuplicates(head.next)
+
+        head.next = self.deleteDuplicates(head.next)
+        return head
+
+```

82. Remove Duplicates from Sorted List II/step1.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
+
+        def get_distinct_list(head_arg):
+            if not head_arg or not head_arg.next:
+                return head_arg
+            if head_arg.val == head_arg.next.val:
+                while head_arg.next and head_arg.val == head_arg.next.val:
+                    head_arg = head_arg.next
+                head_arg = head_arg.next
+                return get_distinct_list(head_arg)
+
+            head_arg.next = get_distinct_list(head_arg.next)
+            return head_arg
+
+        return get_distinct_list(head)

82. Remove Duplicates from Sorted List II/step2.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
+
+        if not head or not head.next:
+            return head
+
+        if head.val == head.next.val:
+            while head.next and head.val == head.next.val:
+                head = head.next
+            head = head.next
+            return self.deleteDuplicates(head)
+
+        head.next = self.deleteDuplicates(head.next)
+        return head

82. Remove Duplicates from Sorted List II/step3.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head or not head.next:
+            return head
+
+        if head.val == head.next.val:
+            while head.next and head.val == head.next.val:
+                head = head.next
+            return self.deleteDuplicates(head.next)
+
+        head.next = self.deleteDuplicates(head.next)
+        return head