Merge Two Sorted Lists?envtype=problem List V2&envid=rab78cw1

0021.Merge-Two-Sorted-Lists/memo.md

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+# 21. Merge Two Sorted Lists
+
+## step1
+
+10m 未満で解けた
+
+## step2
+解くだけなら簡単だが、ロジックが流暢かどうか
+
+https://github.com/colorbox/leetcode/pull/5
+
+破壊的な解法
+
+> これも、
+> ListNode * smaller = list1;
+> ListNode * bigger = list2;
+> としてからやっていたらそんなに違和感がないと思うんですよ。
+>         while(smaller && bigger){
+>             if(smaller->val > bigger->val){
+>                 swap(smaller, bigger);
+>             }
+>             current->next = smaller;
+>             current = current->next;
+>             smaller = smaller->next;
+>         }
+
+`smaller`, `bigger` という作業用ポインタに置き換えると分かりやすい。
+
+片方が尽きたらそのまま後ろにつなげばよい。

0021.Merge-Two-Sorted-Lists/step1.py

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+from typing import Optional
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def mergeTwoLists(
+        self, list1: Optional[ListNode], list2: Optional[ListNode]
+    ) -> Optional[ListNode]:
+        node1 = list1
+        node2 = list2
+        dummy = ListNode()
+        merged = dummy
+
+        while node1 is not None or node2 is not None:
+            if node1 is None:
+                node1, node2 = node2, node1
+            if node2 is None or node1.val < node2.val:
+                merged.next = ListNode(val=node1.val)
+                node1 = node1.next
+            else:
+                merged.next = ListNode(val=node2.val)
+                node2 = node2.next
+            merged = merged.next
+
+        return dummy.next

0021.Merge-Two-Sorted-Lists/step2.py

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+from typing import Optional
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def mergeTwoLists(
+        self, list1: Optional[ListNode], list2: Optional[ListNode]
+    ) -> Optional[ListNode]:
+        sentinel = ListNode()
+        node = sentinel
+
+        node1 = list1
+        node2 = list2
+        while node1 is not None and node2 is not None:
+            if node1.val <= node2.val:
+                node.next = node1
+                node1 = node1.next
+            else:
+                node.next = node2
+                node2 = node2.next
+            node = node.next
+
+        node.next = node1 if node1 is not None else node2
+        return sentinel.next
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def mergeTwoLists(
+        self, list1: Optional[ListNode], list2: Optional[ListNode]
+    ) -> Optional[ListNode]:
+        sentinel = ListNode()
+        node = sentinel
+
+        node1 = list1
+        node2 = list2
+        while node1 is not None and node2 is not None:
+            if node1.val <= node2.val:
+                node.next = node1
+                node1 = node1.next
+            else:
+                node.next = node2
+                node2 = node2.next
+            node = node.next
+
+        node.next = node1 if node1 is not None else node2
+        return sentinel.next