diff --git a/2_AddTwoNumbers/2_AddTwoNumbers.cpp b/2_AddTwoNumbers/2_AddTwoNumbers.cpp
new file mode 100644
index 0000000..4e45432
--- /dev/null
+++ b/2_AddTwoNumbers/2_AddTwoNumbers.cpp
@@ -0,0 +1,144 @@
+// data_structures
+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) {}
+};
+
+// Step1
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy = ListNode(0, nullptr);
+        ListNode *node = &dummy;
+
+        bool carryUp = false;
+
+        while (l1 != nullptr && l2 != nullptr) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            int sum = l1->val + l2->val;
+            if (carryUp == true) {
+                sum++;
+            }
+            
+            if (sum >= 10) {
+                carryUp = true;
+            } else {
+                carryUp = false;
+            }
+
+            node->val = sum % 10;
+            l1 = l1->next;
+            l2 = l2->next;
+        }
+        while (l1 != nullptr) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            int sum = l1->val;
+            if (carryUp == true) {
+                sum++;
+            }
+            node->val = sum % 10;
+            
+            if (sum >= 10) {
+                carryUp = true;
+            } else {
+                carryUp = false;
+            }
+
+            l1 = l1->next;
+        }
+        while (l2 != nullptr) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            int sum = l2->val;
+            if (carryUp == true) {
+                sum++;
+            }
+            node->val = sum % 10;
+            
+            if (sum >= 10) {
+                carryUp = true;
+            } else {
+                carryUp = false;
+            }
+
+            l2 = l2->next;
+        }
+        if (carryUp == true) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            node->val = 1;
+        }
+        return dummy.next;
+    }
+};
+
+// Step2
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy(0, nullptr);
+        ListNode *node = &dummy;
+        bool carryUp = false;
+
+        while (l1 != nullptr || l2 != nullptr || carryUp == true) {
+            const int x = (l1 != nullptr) ? l1->val : 0;
+            const int y = (l2 != nullptr) ? l2->val : 0;
+            
+            int sum = x + y;
+            if (carryUp == true) {
+                sum++;
+            }
+
+            node->next = new ListNode((sum % 10), nullptr);
+            node = node->next;
+            carryUp = sum >= 10 ? true : false;
+
+            if (l1 != nullptr) {
+                l1 = l1->next;
+            }
+            if (l2 != nullptr) {
+                l2 = l2->next;
+            }
+        }
+        return dummy.next;
+    }
+};
+
+// Step3
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy = ListNode(0);
+        ListNode *node = &dummy;
+
+        int carryUp = 0;
+
+        while (l1 != nullptr || l2 != nullptr || carryUp != 0) {
+            const int x = (l1 != nullptr) ? l1->val : 0;
+            const int y = (l2 != nullptr) ? l2->val : 0;
+            const int sum = x + y + carryUp;
+
+            node->next = new ListNode(sum % 10);
+            node = node->next;
+            
+            carryUp = sum/10;
+
+            if (l1 != nullptr) {
+                l1 = l1->next;
+            }
+            if (l2 != nullptr) {
+                l2 = l2->next;
+            }
+        }
+        return dummy.next;
+    }
+};
diff --git a/2_AddTwoNumbers/2_AddTwoNumbers.md b/2_AddTwoNumbers/2_AddTwoNumbers.md
new file mode 100644
index 0000000..b37a3fa
--- /dev/null
+++ b/2_AddTwoNumbers/2_AddTwoNumbers.md
@@ -0,0 +1,178 @@
+# 2. Add Two Numbers
+- [Arai60](https://1kohei1.com/leetcode/)
+- 問題文: [2. Add Two Numbers](https://leetcode.com/problems/add-two-numbers/description/)
+- 使用言語: C++
+- 次に解く問題: [20. Valid Parentheses](https://leetcode.com/problems/valid-parentheses/description/)
+
+## 知識セット
+### 知っていたこと
+- 連結リストの扱い方
+- boolの扱い方
+
+### 今回調べたこと
+- 三項間演算子の使い所
+	- constへの代入、単純なA/B選択などで可読性を落とさず使える
+- boolのメリットデメリット
+	- メリット: 可読性が高い
+	- デメリット: 処理のためにif文を介さないといけないため実行速度が落ちる
+
+## Step1
+- 所要時間: 30min.
+- 方針: returnのための新しい連結リストを作る
+	- l1とl2のvalを足し合わせてsumを求める
+	- carryUp(繰り上がり)をメモしておいてインクリメント
+- コード
+```cpp
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy = ListNode(0, nullptr);
+        ListNode *node = &dummy;
+
+        bool carryUp = false;
+
+        while (l1 != nullptr && l2 != nullptr) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            int sum = l1->val + l2->val;
+            if (carryUp == true) {
+                sum++;
+            }
+            
+            if (sum >= 10) {
+                carryUp = true;
+            } else {
+                carryUp = false;
+            }
+
+            node->val = sum % 10;
+            l1 = l1->next;
+            l2 = l2->next;
+        }
+        while (l1 != nullptr) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            int sum = l1->val;
+            if (carryUp == true) {
+                sum++;
+            }
+            node->val = sum % 10;
+            
+            if (sum >= 10) {
+                carryUp = true;
+            } else {
+                carryUp = false;
+            }
+
+            l1 = l1->next;
+        }
+        while (l2 != nullptr) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            int sum = l2->val;
+            if (carryUp == true) {
+                sum++;
+            }
+            node->val = sum % 10;
+            
+            if (sum >= 10) {
+                carryUp = true;
+            } else {
+                carryUp = false;
+            }
+
+            l2 = l2->next;
+        }
+        if (carryUp == true) {
+            node->next = new ListNode(0, nullptr);
+            node = node->next;
+
+            node->val = 1;
+        }
+        return dummy.next;
+    }
+};
+```
+
+## Step2
+- 所要時間: 15min.
+- 方針: 重複した処理が3つ続いてしまったので関数化もしくは条件分岐で処理
+	- 関数化は結局内部で条件分岐が必要だから、あんまり意味ないので条件分岐で実装
+	- 三項間演算子を活用
+	- constは積極的に使ったほうが可読性高いかも
+- コード
+```cpp
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy(0, nullptr);
+        ListNode *node = &dummy;
+        bool carryUp = false;
+
+        while (l1 != nullptr || l2 != nullptr || carryUp == true) {
+            const int x = (l1 != nullptr) ? l1->val : 0;
+            const int y = (l2 != nullptr) ? l2->val : 0;
+            
+            int sum = x + y;
+            if (carryUp == true) {
+                sum++;
+            }
+
+            node->next = new ListNode((sum % 10), nullptr);
+            node = node->next;
+            carryUp = sum >= 10 ? true : false;
+
+            if (l1 != nullptr) {
+                l1 = l1->next;
+            }
+            if (l2 != nullptr) {
+                l2 = l2->next;
+            }
+        }
+        return dummy.next;
+    }
+};
+```
+
+## Step3 リファクタリング
+- 所要時間: 10min
+- 方針: 
+	- boolではなくintにしてif文を減らす
+	- コンストラクタのデフォルト引数を活用する
+- コード
+```cpp
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy = ListNode(0);
+        ListNode *node = &dummy;
+
+        int carryUp = 0;
+
+        while (l1 != nullptr || l2 != nullptr || carryUp != 0) {
+            const int x = (l1 != nullptr) ? l1->val : 0;
+            const int y = (l2 != nullptr) ? l2->val : 0;
+            const int sum = x + y + carryUp;
+
+            node->next = new ListNode(sum % 10);
+            node = node->next;
+            
+            carryUp = sum/10;
+
+            if (l1 != nullptr) {
+                l1 = l1->next;
+            }
+            if (l2 != nullptr) {
+                l2 = l2->next;
+            }
+        }
+        return dummy.next;
+    }
+};
+```
+- 計算量
+	- 時間計算量: O(N)
+	- 空間計算量: O(N)