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+# 2_add-two-numbers.md
+
+## 1st
+
+所要時間: 17:12
+
+n: l1, l2それぞれのノード数の大きい方
+- 時間計算量: O(n)
+- 空間計算量: O(n)
+
+一度解いたことがあるにしては時間がかかりすぎている。 `l1 || l2 || curry` のorをandと間違えて1ミス、解消に手間取る。
+
+- 変数名addedはadded_value_nodeか。単にnodeでも十分かも。先頭がdummyで最初はdummyを指すので名前がつけづらい
+- while内のvalという変数名はtotalくらいか。個人的にはvalを算出する計算が分かりやすいので許容範囲かという気はする
+
+```cpp
+/**
+ * Definition for singly-linked list.
+ * 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* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode* dummy_head = new ListNode(0);
+        ListNode* added = dummy_head;
+        int curry = 0;
+        while (l1 || l2 || curry) {
+            const int val = getValueOrDefault(l1) + getValueOrDefault(l2) + curry;
+            const int digit = val % 10;
+            curry = val / 10;
+            added->next = new ListNode(digit);
+            added = added->next;
+            l1 = getNextIfExists(l1);
+            l2 = getNextIfExists(l2);
+        }
+        return dummy_head->next;
+    }
+
+private:
+    static int getValueOrDefault(const ListNode* const node) {
+        if (!node) return 0;
+        return node->val;
+    }
+
+    static ListNode* getNextIfExists(const ListNode* const node) {
+        if (!node) return nullptr;
+        return node->next;
+    }
+};
+```
+
+## 2nd
+
+### 参考
+
+- https://discord.com/channels/1084280443945353267/1231966485610758196/1237404231225577592
+- https://discord.com/channels/1084280443945353267/1225849404037009609/1233844967777243318
+- https://discord.com/channels/1084280443945353267/1233603535862628432/1233768573718106253
+- https://discord.com/channels/1084280443945353267/1200089668901937312/1206949445359640576
+
+再帰バージョンで書いた。
+
+所要時間: 10:12
+
+n: l1, l2それぞれのノード数の大きい方
+- 時間計算量: O(n)
+- 空間計算量: O(n)
+
+リストのサイズが高々100なのでスタックオーバーフローすることはないだろう。
+スタックサイズ8MBとして、スタックフレーム辺りの引数、ローカル変数でポインタ3つ、intが2つ + ベースポインタとリターンアドレスで48byte+αくらいだろうか？とすると 8*2**20/48 ≒ 170K回くらいは深くできそう。
+(ポインタやレジスタのサイズを64bitとして計算している)
+
+```cpp
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        return addTwoNumbersHelper(l1, l2, 0);
+    }
+
+private:
+    static ListNode* addTwoNumbersHelper(
+        const ListNode* const digit_node1,
+        const ListNode* const digit_node2,
+        const int curry
+    ) {
+        if (!digit_node1 && !digit_node2 && !curry) return nullptr;
+        ListNode* const digit_node = new ListNode(0);
+        const int total = getValOrDefault(digit_node1) + getValOrDefault(digit_node2) + curry;
+        digit_node->val = total % 10; // digit
+        digit_node->next = addTwoNumbersHelper(getNextIfExists(digit_node1), getNextIfExists(digit_node2), total / 10);
+        return digit_node;
+    }
+
+    static int getValOrDefault(const ListNode* const node) {
+        if (!node) return 0;
+        return node->val;
+    }
+
+    static ListNode* getNextIfExists(const ListNode* const node) {
+        if (!node) return nullptr;
+        return node->next;
+    }
+};
+```
+
+
+## 3rd
+
+
+```cpp
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy_head = ListNode();
+        ListNode* node = &dummy_head;
+        int curry = 0;
+        while (l1 || l2 || curry) {
+            const int total = getValOrDefault(l1) + getValOrDefault(l2) + curry;
+            curry = total / 10;
+            node->next = new ListNode(total % 10); // digit
+            node = node->next;
+            l1 = getNextIfExists(l1);
+            l2 = getNextIfExists(l2);
+        }
+        return dummy_head.next;
+    }
+
+private:
+    static int getValOrDefault(const ListNode* const node) {
+        if (!node) return 0;
+        return node->val;
+    }
+
+    static ListNode* getNextIfExists(const ListNode* const node) {
+        if (!node) return nullptr;
+        return node->next;
+    }
+};
+```
+
+## 4th
+
+```cpp
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy_head = ListNode(-1);
+        ListNode* node = &dummy_head;
+        int carry = 0;
+        while (l1 || l2 || carry) {
+            const int total = getValOrDefault(l1, 0) + getValOrDefault(l2, 0) + carry;
+            carry = total / 10;
+            node->next = new ListNode(total % 10); // digit
+            node = node->next;
+            l1 = getNextIfExists(l1);
+            l2 = getNextIfExists(l2);
+        }
+        return dummy_head.next;
+    }
+
+private:
+    static int getValOrDefault(const ListNode* const node, const int default_value) {
+        if (!node) return default_value;
+        return node->val;
+    }
+
+    static ListNode* getNextIfExists(const ListNode* const node) {
+        if (!node) return nullptr;
+        return node->next;
+    }
+};
+```
+
+## 5th
+
+スタックで行きがけの再帰
+
+```cpp
+struct DigitInfo {
+    ListNode* prev_added;
+    ListNode* digit_node1;
+    ListNode* digit_node2;
+    int carry;
+};
+
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode dummy_head = {};
+        stack<DigitInfo> digit_info_stack = {};
+        digit_info_stack.push({&dummy_head, l1, l2, 0});
+        while (!digit_info_stack.empty()) {
+            auto [prev, digit_node1, digit_node2, carry] = digit_info_stack.top();
+            digit_info_stack.pop();
+            if (!digit_node1 && !digit_node2 && !carry) continue;
+            const int total = getValOrDefault(digit_node1, 0) + getValOrDefault(digit_node2, 0) + carry;
+            prev->next = new ListNode(total % 10);
+            digit_info_stack.push({prev->next, getNextIfExists(digit_node1), getNextIfExists(digit_node2), total / 10});
+        }
+        return dummy_head.next;
+    }
+
+private:
+    static int getValOrDefault(const ListNode* const node, const int default_value) {
+        if (!node) return default_value;
+        return node->val;
+    }
+
+    static ListNode* getNextIfExists(const ListNode* const node) {
+        if (!node) return nullptr;
+        return node->next;
+    }
+};
+```
+
+スタックで帰りがけの再帰
+
+```cpp
+enum RecursiveDirection {
+    go, back
+};
+
+struct DigitInfo {
+    ListNode* prev;
+    ListNode* current;
+    ListNode* digit_node1;
+    ListNode* digit_node2;
+    int carry;
+};
+
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode* head = new ListNode(0);
+        stack<pair<RecursiveDirection, DigitInfo>> digit_info_stack = {};
+        digit_info_stack.push({RecursiveDirection::go, {nullptr, head, l1, l2, 0}});
+        while (!digit_info_stack.empty()) {
+            auto [direction, info] = digit_info_stack.top();
+            auto [prev, current, digit_node1, digit_node2, carry] = info;
+            digit_info_stack.pop();
+            if (direction == RecursiveDirection::go) {
+                if (!digit_node1 && !digit_node2 && !carry) continue;
+                int total = getValOrDefault(digit_node1, 0) + getValOrDefault(digit_node2, 0) + carry;
+                current->val = total % 10;
+                digit_info_stack.push({RecursiveDirection::back, {prev, current, nullptr, nullptr, 0}});
+                digit_info_stack.push({
+                    RecursiveDirection::go,
+                    {current, new ListNode(), getNextIfExists(digit_node1), getNextIfExists(digit_node2), total / 10}
+                });
+            } else {
+                // back
+                if (prev) {
+                    prev->next = current;
+                }
+            }
+        }
+        return head;
+    }
+
+private:
+    static int getValOrDefault(const ListNode* const node, const int default_value) {
+        if (!node) return default_value;
+        return node->val;
+    }
+
+    static ListNode* getNextIfExists(const ListNode* const node) {
+        if (!node) return nullptr;
+        return node->next;
+    }
+};
+```
+
+## 6th
+
+帰りがけstack版。enumはboolで十分かもしれないが、一応。
+
+```cpp
+enum class RecursiveDirection {
+    kGo, kBack
+};
+
+struct DigitInfo {
+    RecursiveDirection direction;
+    ListNode*& prev;
+    ListNode* node;
+    int digit;
+    ListNode* l1;
+    ListNode* l2;
+    int carry;
+};
+
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        ListNode* head = nullptr;
+        stack<DigitInfo> digit_info_stack = {};
+        digit_info_stack.push({RecursiveDirection::kGo, head, nullptr, 0, l1, l2, 0});
+        while (!digit_info_stack.empty()) {
+            auto& [direction, prev, node, digit, l1, l2, carry] = digit_info_stack.top();
+            if (direction == RecursiveDirection::kBack) {
+                prev = new ListNode(digit, node);
+                digit_info_stack.pop();
+                continue;
+            }
+            if (!l1 && !l2 && !carry) {
+                digit_info_stack.pop();
+                continue;
+            }
+            int total = carry;
+            if (l1) {
+                total += l1->val;
+                l1 = l1->next;
+            }
+            if (l2) {
+                total += l2->val;
+                l2 = l2->next;
+            }
+            digit = total % 10;
+            direction = RecursiveDirection::kBack;
+            digit_info_stack.push({RecursiveDirection::kGo, node, nullptr, 0, l1, l2, total / 10});
+        }
+        return head;
+    }
+};
+```