2. Add Two Numbers

24_2.py

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+## ２.  Add Two Numbers
+### 24問目
+### [問題リンク](https://leetcode.com/problems/add-two-numbers/description/?envType=problem-list-v2&envId=ryq3bf2c)
+
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+
+#1st:書ききれずに解答を見る
+#2nd:13min
+#3rd:4min
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        dummy_head = ListNode()
+        tail = dummy_head
+        carry = 0
+
+        while l1 or l2 or carry != 0:
+            l1_val = l1.val if l1 else 0
+            l2_val = l2.val if l2 else 0
+            total = l1_val + l2_val + carry
+            carry = total // 10
+
+            next_node = ListNode(total % 10)
+            tail.next = next_node
+            tail = tail.next
+
+            l1 = l1.next if l1 else None
+            l2 = l2.next if l2 else None
+
+        return dummy_head.next
+
+### 参考を見て書いた他方法
+#再帰を使う方法
+#dummy_headを使わない方法
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        carry = 0
+        def add_two_num_helper(l1, l2, carry):
+            if l1 is None and l2 is None and carry == 0:
+                return None
+
+            l1_val = l1.val if l1 else 0
+            l2_val = l2.val if l2 else 0
+            total = l1_val + l2_val + carry
+            carry = total // 10
+
+            new_node = ListNode(total % 10)
+            sum_node_head = new_node
+            l1 = l1.next if l1 else None
+            l2 = l2.next if l2 else None
+            sum_node_head.next = add_two_num_helper(l1, l2, carry)
+            return sum_node_head
+        return add_two_num_helper(l1, l2, carry)
+
+'''
+参考にしたもの
+* https://github.com/cheeseNA/leetcode/pull/2
+→ 再帰を使った方法
+→ python3のintはdoubleに相当し、その上限はメモリに依存する　→
+[python docのint概要](https://docs.python.org/ja/3/library/functions.html#int)
+→ int()の第二引数で基数を指定することで、異なる基数の値も整数値への変換もできる。文字列も可能。
+→ 逆に2,8,16進数への変換は bin(), oct(), hex()
+'''