diff --git a/0721.Accounts-Merge/memo.md b/0721.Accounts-Merge/memo.md
new file mode 100644
index 0000000..36588a6
--- /dev/null
+++ b/0721.Accounts-Merge/memo.md
@@ -0,0 +1,53 @@
+# 721. Accounts Merge
+
+## step1
+40m ぐらいかかった。UnionFindは記憶に自信がなく少し調べてしまった。
+同値類を管理するデータ構造としてこれが素直な方針だと個人的には思った
+
+revised: 変数名を改善
+
+
+## 他の人のコード
+https://github.com/huyfififi/coding-challenges/pull/48
+
+```python
+for i, (_, *emails) in enumerate(accounts):
+```
+この書き方は知らなかった。
+
+https://peps.python.org/pep-3132/
+
+> This PEP proposes a change to iterable unpacking syntax, allowing to specify a “catch-all” name which will be assigned a list of all items not assigned to a “regular” name.
+
+```python
+[accounts[i][0]] + sorted(emails)
+```
+より
+```python
+[accounts[i][0], *sorted(emails)]
+```
+の方がlistの生成回数が少ない
+
+自分はnameで分離した後にunion-findで同値類を求めたがnameで分離しない方が自然だな。
+
+DFSでも書ける、なるほど。
+
+## step2
+DFSとunion findで書き直し
+
+時間計算量：
+
+N = accounts数、E = 全email出現回数、U = ユニークemail数
+
+DFS: O(E+UlogU)
+
+UnionFind: O(E\alpha(N)+UlogU) \alpha: 逆アッカーマン関数
+
+## C++
+LLMのコードを写経する形になった。自分ではまだ書けない。
+
+- unionは共用体を表す予約語
+    - 共用体：すべてのメンバ変数が、メモリ上の同じ場所を共有する
+- std::setは赤黒木で管理されているので重複排除とソートを同時に行える
+    - https://ja.wikipedia.org/wiki/%E8%B5%A4%E9%BB%92%E6%9C%A8
+
diff --git a/0721.Accounts-Merge/step1.py b/0721.Accounts-Merge/step1.py
new file mode 100644
index 0000000..daeda2c
--- /dev/null
+++ b/0721.Accounts-Merge/step1.py
@@ -0,0 +1,61 @@
+class UnionFind:
+    def __init__(self, n):
+        self.parent = list(range(n))
+        self.size = [1] * n
+
+    def find(self, x):
+        if x != self.parent[x]:
+            self.parent[x] = self.find(self.parent[x])
+        return self.parent[x]
+
+    def union(self, x, y):
+        parent_x = self.find(x)
+        parent_y = self.find(y)
+
+        if parent_x == parent_y:
+            return parent_x
+
+        if self.size[parent_x] < self.size[parent_y]:
+            parent_x, parent_y = parent_y, parent_x
+
+        self.parent[parent_y] = parent_x
+        self.size[parent_x] += self.size[parent_y]
+
+
+class Solution:
+    def accountsMerge(self, accounts: list[list[str]]) -> list[list[str]]:
+        def merge_accounts_with_same_name(name, indexes):
+            if len(indexes) == 1:
+                return [
+                    [accounts[indexes[0]][0]] + sorted(set(accounts[indexes[0]][1:]))
+                ]
+
+            mail_to_rank = {}
+            account_union_find = UnionFind(len(indexes))
+            for r, i in enumerate(indexes):
+                for email in accounts[i][1:]:
+                    if email in mail_to_rank:
+                        account_union_find.union(r, mail_to_rank[email])
+                    else:
+                        mail_to_rank[email] = r
+            rank_to_index = {r: i for r, i in enumerate(indexes)}
+            root_to_emails = {}
+            for r in range(len(indexes)):
+                root = account_union_find.find(r)
+                root_to_emails.setdefault(root, []).extend(
+                    accounts[rank_to_index[r]][1:]
+                )
+            result = []
+            for emails in root_to_emails.values():
+                result.append([name] + sorted(set(emails)))
+            return result
+
+        name_to_indexes = {}
+        for i, account in enumerate(accounts):
+            name_to_indexes.setdefault(account[0], []).append(i)
+
+        accounts_merged = []
+        for name, indexes in name_to_indexes.items():
+            accounts_merged.extend(merge_accounts_with_same_name(name, indexes))
+
+        return accounts_merged
diff --git a/0721.Accounts-Merge/step1_revised.py b/0721.Accounts-Merge/step1_revised.py
new file mode 100644
index 0000000..bab8e32
--- /dev/null
+++ b/0721.Accounts-Merge/step1_revised.py
@@ -0,0 +1,67 @@
+class UnionFind:
+    def __init__(self, n):
+        self.parent = list(range(n))
+        self.size = [1] * n
+
+    def find(self, x):
+        if x != self.parent[x]:
+            self.parent[x] = self.find(self.parent[x])
+        return self.parent[x]
+
+    def union(self, x, y):
+        parent_x = self.find(x)
+        parent_y = self.find(y)
+
+        if parent_x == parent_y:
+            return parent_x
+
+        if self.size[parent_x] < self.size[parent_y]:
+            parent_x, parent_y = parent_y, parent_x
+
+        self.parent[parent_y] = parent_x
+        self.size[parent_x] += self.size[parent_y]
+
+
+class Solution:
+    def accountsMerge(self, accounts: list[list[str]]) -> list[list[str]]:
+        def merge_accounts_with_same_name(name, account_indexes):
+            if len(account_indexes) == 1:
+                return [
+                    [accounts[account_indexes[0]][0]]
+                    + sorted(set(accounts[account_indexes[0]][1:]))
+                ]
+
+            mail_to_pos = {}
+            account_union_find = UnionFind(len(account_indexes))
+            for account_pos, account_index in enumerate(account_indexes):
+                for email in accounts[account_index][1:]:
+                    if email in mail_to_pos:
+                        account_union_find.union(account_pos, mail_to_pos[email])
+                    else:
+                        mail_to_pos[email] = account_pos
+            pos_to_account_index = {
+                account_pos: account_index
+                for account_pos, account_index in enumerate(account_indexes)
+            }
+            root_to_emails = {}
+            for account_pos in range(len(account_indexes)):
+                root = account_union_find.find(account_pos)
+                root_to_emails.setdefault(root, []).extend(
+                    accounts[pos_to_account_index[account_pos]][1:]
+                )
+            result = []
+            for emails in root_to_emails.values():
+                result.append([name] + sorted(set(emails)))
+            return result
+
+        name_to_account_indexes = {}
+        for account_index, account in enumerate(accounts):
+            name_to_account_indexes.setdefault(account[0], []).append(account_index)
+
+        accounts_merged = []
+        for name, account_indexes in name_to_account_indexes.items():
+            accounts_merged.extend(
+                merge_accounts_with_same_name(name, account_indexes)
+            )
+
+        return accounts_merged
diff --git a/0721.Accounts-Merge/step2_dfs.py b/0721.Accounts-Merge/step2_dfs.py
new file mode 100644
index 0000000..c0dcbcb
--- /dev/null
+++ b/0721.Accounts-Merge/step2_dfs.py
@@ -0,0 +1,36 @@
+class Solution:
+    def accountsMerge(self, accounts: List[List[str]]) -> List[List[str]]:
+        email_to_name = {}
+        email_to_neighbor = {}
+        hubs = []
+        for name, hub, *rest in accounts:
+            email_to_name[hub] = name
+            hubs.append(hub)
+            for email in rest:
+                email_to_name[email] = name
+                email_to_neighbor.setdefault(hub, []).append(email)
+                email_to_neighbor.setdefault(email, []).append(hub)
+
+        accounts_merged = []
+        visited = set()
+
+        def traverse(email_start):
+            if email_start in visited:
+                return
+
+            visited.add(email_start)
+            stack = [email_start]
+            emails = [email_start]
+            while stack:
+                email = stack.pop()
+                for neighbor in email_to_neighbor.get(email, []):
+                    if neighbor not in visited:
+                        stack.append(neighbor)
+                        emails.append(neighbor)
+                        visited.add(neighbor)
+            accounts_merged.append([email_to_name[email_start], *sorted(set(emails))])
+
+        for email_start in hubs:
+            traverse(email_start)
+
+        return accounts_merged
diff --git a/0721.Accounts-Merge/step2_union_find.cpp b/0721.Accounts-Merge/step2_union_find.cpp
new file mode 100644
index 0000000..e5100ff
--- /dev/null
+++ b/0721.Accounts-Merge/step2_union_find.cpp
@@ -0,0 +1,83 @@
+#include <vector>
+#include <string>
+#include <unordered_map>
+#include <set>
+#include <numeric>
+#include <algorithm>
+
+class UnionFind {
+public:
+    std::vector<int> parent;
+    std::vector<int> size;
+
+    UnionFind(int n) {
+        parent.resize(n);
+        std::iota(parent.begin(), parent.end(), 0);
+        size.assign(n, 1);
+    }
+
+    int find(int x) {
+        if (x != parent[x]) {
+            parent[x] = find(parent[x]);
+        }
+        return parent[x];
+    }
+
+    void union_op(int x, int y) {
+        int parent_x = find(x);
+        int parent_y = find(y);
+
+        if (parent_x == parent_y) {
+            return;
+        }
+
+        if (size[parent_x] < size[parent_y]) {
+            std::swap(parent_x, parent_y);
+        }
+
+        parent[parent_y] = parent_x;
+        size[parent_x] += size[parent_y];
+        return;
+    }
+};
+
+class Solution {
+public:
+    std::vector<std::vector<std::string>> accountsMerge(std::vector<std::vector<std::string>>& accounts) {
+        UnionFind union_find(accounts.size());
+        std::unordered_map<std::string, int> mail_to_account_index;
+        std::vector<std::vector<std::string>> accounts_merged;
+
+        for (int account_index = 0; account_index < accounts.size(); ++account_index) {
+            for (size_t i = 1; i < accounts[account_index].size(); ++i) {
+                const std::string& email = accounts[account_index][i];
+                if (mail_to_account_index.count(email)) {
+                    union_find.union_op(account_index, mail_to_account_index[email]);
+                } else {
+                    mail_to_account_index[email] = account_index;
+                }
+            }
+        }
+
+        std::unordered_map<int, std::vector<std::string>> root_to_emails;
+        for (int account_index = 0; account_index < accounts.size(); ++account_index) {
+            int root = union_find.find(account_index);
+            for (size_t i = 1; i < accounts[account_index].size(); ++i) {
+                root_to_emails[root].push_back(accounts[account_index][i]);
+            }
+        }
+
+        for (auto& [root, emails] : root_to_emails) {
+            std::set<std::string> unique_emails(emails.begin(), emails.end());
+
+            std::vector<std::string> merged_row;
+            merged_row.push_back(accounts[root][0]);
+            for (const auto& email : unique_emails) {
+                merged_row.push_back(email);
+            }
+            accounts_merged.push_back(merged_row);
+        }
+
+        return accounts_merged;
+    }
+};
diff --git a/0721.Accounts-Merge/step2_union_find.py b/0721.Accounts-Merge/step2_union_find.py
new file mode 100644
index 0000000..f014027
--- /dev/null
+++ b/0721.Accounts-Merge/step2_union_find.py
@@ -0,0 +1,46 @@
+class UnionFind:
+    def __init__(self, n):
+        self.parent = list(range(n))
+        self.size = [1] * n
+
+    def find(self, x):
+        if x != self.parent[x]:
+            self.parent[x] = self.find(self.parent[x])
+        return self.parent[x]
+
+    def union(self, x, y):
+        parent_x = self.find(x)
+        parent_y = self.find(y)
+
+        if parent_x == parent_y:
+            return parent_x
+
+        if self.size[parent_x] < self.size[parent_y]:
+            parent_x, parent_y = parent_y, parent_x
+
+        self.parent[parent_y] = parent_x
+        self.size[parent_x] += self.size[parent_y]
+
+
+class Solution:
+    def accountsMerge(self, accounts: list[list[str]]) -> list[list[str]]:
+        union_find = UnionFind(len(accounts))
+        mail_to_account_index = {}
+        accounts_merged = []
+        for account_index, (_, *emails) in enumerate(accounts):
+            for email in emails:
+                if email in mail_to_account_index:
+                    union_find.union(account_index, mail_to_account_index[email])
+                else:
+                    mail_to_account_index[email] = account_index
+
+        root_to_emails = {}
+        for account_index in range(len(accounts)):
+            root = union_find.find(account_index)
+            root_to_emails.setdefault(root, []).extend(accounts[account_index][1:])
+
+        accounts_merged = []
+        for root, emails in root_to_emails.items():
+            accounts_merged.append([accounts[root][0], *sorted(set(emails))])
+
+        return accounts_merged