Create 2. Add Two Numbers.md

競技プロ就活部PR用/2. Add Two Numbers.md

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+## whileによる解法
+* 時間計算量: O(N)
+* 空間計算量: O(N)
+
+### 1回目 (14m25s)
+* ビット演算で似た問題を解いていたので、指針は思いついた。
+* 最初のノードの計算を特別扱いしているが、これは無くせそう。
+
+
+```python
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        dummy = ListNode((l1.val + l2.val) % 10)
+        dummy_head = dummy
+        carry = (l1.val + l2.val) // 10
+
+        current1 = l1
+        current2 = l2
+
+        if current1: current1 = current1.next
+        if current2: current2 = current2.next
+
+        while current1 or current2 or carry:
+            val1 = current1.val if current1 else 0
+            val2 = current2.val if current2 else 0
+
+            dummy.next = ListNode((val1 + val2 + carry) % 10)
+            dummy = dummy.next
+            carry = (val1 + val2 + carry) // 10
+
+            if current1: current1 = current1.next
+            if current2: current2 = current2.next
+
+        return dummy_head
+```
+
+### 2回目
+* 1回目の計算の特別扱いを修正。
+* dummyの初期化時に0を明示しなくても良さそう。
+```python
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        dummy = ListNode()
+        dummy_head = dummy
+        carry = 0
+        current1 = l1
+        current2 = l2
+
+        while current1 or current2 or carry:
+            val1 = current1.val if current1 else 0
+            val2 = current2.val if current2 else 0
+
+            dummy.next = ListNode((val1 + val2 + carry) % 10)
+            dummy = dummy.next
+            carry = (val1 + val2 + carry) // 10
+
+            if current1: current1 = current1.next
+            if current2: current2 = current2.next
+
+        return dummy_head.next
+```
+
+
+### 3回目
+他参加者の参考になった点として、
+* val1 = current1.val if current1 else 0の部分をget_valueとして関数化
+* dummyの変数名を代わりにsentinelにする。
+* totalでval1 + val2 + carryを変数化
+を採用してみる。
+
+```python
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        def get_value(node: Optional[ListNode]) -> int:
+            if node is None:
+                return 0
+            return node.val
+
+        sentinel = ListNode()
+        sentinel_head = sentinel
+        carry = 0
+
+        while l1 or l2 or carry:
+            value1 = get_value(l1)
+            value2 = get_value(l2)
+
+            total = value1 + value2 + carry
+            sentinel.next = ListNode(total % 10)
+            sentinel = sentinel.next
+            carry = total // 10
+
+            if l1: l1 = l1.next
+            if l2: l2 = l2.next
+
+        return sentinel_head.next
+```
+
+
+## 再帰による解法
+* 比較的すんなり書き換えられた。
+
+```python
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        def get_value(node: Optional[ListNode]) -> int:
+            if node is None:
+                return 0
+            return node.val
+
+        def next_node(node: Optional[ListNode]) -> Optional[ListNode]:
+            if node is None:
+                return None
+            return node.next
+
+        def _add_two_numbers_helper(node1: Optional[ListNode] , node2: Optional[ListNode], carry: int) -> Optional[ListNode]:
+            if not (node1 or node2 or carry):
+                return None
+
+            value1 = get_value(node1)
+            value2 = get_value(node2)
+            total = value1 + value2 + carry
+            node = ListNode(total % 10)
+            carry = total // 10
+            node.next = _add_two_numbers_helper(next_node(node1), next_node(node2), carry)
+            return node
+
+        return _add_two_numbers_helper(l1, l2, 0)
+```