diff --git a/1.linked-ist/remove-duplicates-from-sorted-list-II/memo.md b/1.linked-ist/remove-duplicates-from-sorted-list-II/memo.md
new file mode 100644
index 0000000..d46dd04
--- /dev/null
+++ b/1.linked-ist/remove-duplicates-from-sorted-list-II/memo.md
@@ -0,0 +1,12 @@
+## step1
+- 出現が1回の要素から成る新しいリストを返す。
+- 時間計算量O(Nlog(N))、空間計算量O(N)。
+- mapのイテレーションは型が明確なのでautoを使う。
+
+## step2
+- 線形に探索し、被りがあればheadを変更すると、時間計算量O(N)、空間計算量O(1)になることに気づき、実装する。
+- 用途などはあまり分からず、パフォーマンス要件の考察は断念
+  - 「複数回呼び出される関数に見えるので、どちらの計算量は少ない方が良い」という当たり前の考察しかできなかった。
+
+## step3
+- 3分ほどで実装。Step2でループごとの変数の動きをしっかり理解したため詰まらず実装できた。
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diff --git a/1.linked-ist/remove-duplicates-from-sorted-list-II/step1.cpp b/1.linked-ist/remove-duplicates-from-sorted-list-II/step1.cpp
new file mode 100644
index 0000000..7e4288b
--- /dev/null
+++ b/1.linked-ist/remove-duplicates-from-sorted-list-II/step1.cpp
@@ -0,0 +1,33 @@
+#include <map>
+
+struct ListNode {
+  int val;
+  ListNode *next;
+  ListNode() : val(0), next(nullptr) {}
+  ListNode(int x) : val(x), next(nullptr) {}
+  ListNode(int x, ListNode *next) : val(x), next(next) {}
+};
+
+class Solution {
+public:
+  ListNode* deleteDuplicates(ListNode* head) {
+    std::map<int, int> cnts;
+    while (head) {
+      cnts[head->val]++;
+      head = head->next;
+    }
+    ListNode *deleted_head = NULL;
+    for (auto [n, c] : cnts) {
+      if (c > 1) continue;
+      if (!head) {
+        deleted_head = new ListNode(n);
+        head = deleted_head;
+      }
+      else {
+        head->next = new ListNode(n);
+        head = head->next;
+      }
+    }
+    return deleted_head;
+  }
+};
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diff --git a/1.linked-ist/remove-duplicates-from-sorted-list-II/step2.cpp b/1.linked-ist/remove-duplicates-from-sorted-list-II/step2.cpp
new file mode 100644
index 0000000..b1fcb12
--- /dev/null
+++ b/1.linked-ist/remove-duplicates-from-sorted-list-II/step2.cpp
@@ -0,0 +1,41 @@
+#include <set>
+
+struct ListNode {
+    int val;
+    ListNode *next;
+    ListNode() : val(0), next(nullptr) {}
+    ListNode(int x) : val(x), next(nullptr) {}
+    ListNode(int x, ListNode *next) : val(x), next(next) {}
+};
+
+class Solution {
+public:
+  ListNode* deleteDuplicates(ListNode* head) {
+    ListNode* seek = head;
+    ListNode* pre = NULL;
+    ListNode* border;
+    int sequence_cnt;
+    while (seek) {
+      border = seek;
+      sequence_cnt = 0;
+      while (border->next && border->val == border->next->val) {
+        border = border->next;
+        sequence_cnt++;
+      }
+      if (sequence_cnt == 0) {
+        pre = seek;
+        seek = seek->next;
+        continue;
+      }
+
+      if (!pre) {
+        seek = head = border->next;
+      }
+      else {
+        pre->next = border->next;
+        seek = border->next;
+      }
+    }
+    return head;
+  }
+};
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diff --git a/1.linked-ist/remove-duplicates-from-sorted-list-II/step3.cpp b/1.linked-ist/remove-duplicates-from-sorted-list-II/step3.cpp
new file mode 100644
index 0000000..c8a1166
--- /dev/null
+++ b/1.linked-ist/remove-duplicates-from-sorted-list-II/step3.cpp
@@ -0,0 +1,41 @@
+#include <stdlib.h>
+
+struct ListNode {
+    int val;
+    ListNode *next;
+    ListNode() : val(0), next(nullptr) {}
+    ListNode(int x) : val(x), next(nullptr) {}
+    ListNode(int x, ListNode *next) : val(x), next(next) {}
+};
+
+class Solution {
+public:
+  ListNode* deleteDuplicates(ListNode* head) {
+    ListNode* seek = head;
+    ListNode* pre = NULL;
+    ListNode* border;
+    int sequence_cnt;
+    while (seek) {
+      border = seek;
+      sequence_cnt = 0;
+      while (border->next && border->val == border->next->val) {
+        border = border->next;
+        sequence_cnt++;
+      }
+      if (sequence_cnt == 0) {
+        pre = seek;
+        seek = seek->next;
+        continue;
+      }
+
+      if (!pre) {
+        seek = head = border->next;
+      }
+      else {
+        pre->next = border->next;
+        seek = border->next;
+      }
+    }
+    return head;
+  }
+};
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