From c84c0b757c742a56254b9eacb635913aef229003 Mon Sep 17 00:00:00 2001 From: tom4649 Date: Wed, 24 Jun 2026 06:10:12 +0900 Subject: [PATCH 1/2] step1,2,3 --- 0143.Reorder-List/memo.md | 32 +++++++++++++ 0143.Reorder-List/step1_list.py | 27 +++++++++++ 0143.Reorder-List/step1_two_pointer.py | 35 ++++++++++++++ .../step1_two_pointer_revised.py | 35 ++++++++++++++ 0143.Reorder-List/step2.py | 47 +++++++++++++++++++ 0143.Reorder-List/step3.py | 45 ++++++++++++++++++ 6 files changed, 221 insertions(+) create mode 100644 0143.Reorder-List/memo.md create mode 100644 0143.Reorder-List/step1_list.py create mode 100644 0143.Reorder-List/step1_two_pointer.py create mode 100644 0143.Reorder-List/step1_two_pointer_revised.py create mode 100644 0143.Reorder-List/step2.py create mode 100644 0143.Reorder-List/step3.py diff --git a/0143.Reorder-List/memo.md b/0143.Reorder-List/memo.md new file mode 100644 index 0000000..cbea894 --- /dev/null +++ b/0143.Reorder-List/memo.md @@ -0,0 +1,32 @@ +# 143. Reorder List + +## step1 +まずlistを使った方法を書いてみる。5mほど。これでは解けたことにならなそうなのでポインタのみを使って計算量を抑えたい。 + +Discussionの以下を見て解法を思いついた。後半を reverse すればいけそう。ヒントなしで解法には辿り着けなかった。 + +> Good practice for most of the important techniques of dealing with linked lists, ie. 2 pointer traversing, reversing, merging, ... + +ここまで17mほど。変数名を改善 + +## 他の人のコード + +https://github.com/thonda28/leetcode/pull/4 + +> ノード数が偶数のとき、真ん中のノードは、左半分の最後と右半分の最初の二つありますよね。どっちを返すかコメントに書いた方が分かりやすいです。 + +たしかに + +- front, back, interleaveの命名 + +自分のformer, latterでもよいとは思った + + +> 最初からなるべく（ちょうどいい粒度の）関数に分けた方がいいのではないでしょうか？普段コードを書く時も、全てのコードをメイン関数に書いてから、関数化したりしないですよね。 + +関数化しようと思わなかったのは反省すべきなのかもしれない + +https://github.com/potrue/leetcode/pull/68 + +## step2 +## step3 diff --git a/0143.Reorder-List/step1_list.py b/0143.Reorder-List/step1_list.py new file mode 100644 index 0000000..9c478af --- /dev/null +++ b/0143.Reorder-List/step1_list.py @@ -0,0 +1,27 @@ +# Definition for singly-linked list. +# class ListNode: +# def __init__(self, val=0, next=None): +# self.val = val +# self.next = next +class Solution: + def reorderList(self, head: Optional[ListNode]) -> None: + if head is None: + return + + ordered_list = [] + node = head + while node is not None: + ordered_list.append(node) + node = node.next + + left = 0 + right = len(ordered_list) - 1 + while left < right: + ordered_list[left].next = ordered_list[right] + left += 1 + if left == right: + break + ordered_list[right].next = ordered_list[left] + right -= 1 + + ordered_list[left].next = None diff --git a/0143.Reorder-List/step1_two_pointer.py b/0143.Reorder-List/step1_two_pointer.py new file mode 100644 index 0000000..fe5dd92 --- /dev/null +++ b/0143.Reorder-List/step1_two_pointer.py @@ -0,0 +1,35 @@ +# Definition for singly-linked list. +# class ListNode: +# def __init__(self, val=0, next=None): +# self.val = val +# self.next = next +class Solution: + def reorderList(self, head: Optional[ListNode]) -> None: + if head is None: + return + + slow = head + fast = head + while fast and fast.next: + slow = slow.next + fast = fast.next.next + + original_latter = slow.next + slow.next = None + node = None + + while original_latter is not None: + temp = original_latter.next + original_latter.next = node + node = original_latter + original_latter = temp + + original_former = head + original_latter = node + while original_former is not None and original_latter is not None: + next_former = original_former.next + next_latter = original_latter.next + original_former.next = original_latter + original_latter.next = next_former + original_former = next_former + original_latter = next_latter diff --git a/0143.Reorder-List/step1_two_pointer_revised.py b/0143.Reorder-List/step1_two_pointer_revised.py new file mode 100644 index 0000000..9df2ad9 --- /dev/null +++ b/0143.Reorder-List/step1_two_pointer_revised.py @@ -0,0 +1,35 @@ +# Definition for singly-linked list. +# class ListNode: +# def __init__(self, val=0, next=None): +# self.val = val +# self.next = next +class Solution: + def reorderList(self, head: Optional[ListNode]) -> None: + if head is None: + return + + slow = head + fast = head + while fast and fast.next: + slow = slow.next + fast = fast.next.next + + latter = slow.next + slow.next = None + node = None + + while latter is not None: + next_latter = latter.next + latter.next = node + node = latter + latter = next_latter + + former = head + latter = node + while former is not None and latter is not None: + next_former = former.next + next_latter = latter.next + former.next = latter + latter.next = next_former + former = next_former + latter = next_latter diff --git a/0143.Reorder-List/step2.py b/0143.Reorder-List/step2.py new file mode 100644 index 0000000..4d86c8b --- /dev/null +++ b/0143.Reorder-List/step2.py @@ -0,0 +1,47 @@ +# Definition for singly-linked list. +class ListNode: + def __init__(self, val=0, next=None): + self.val = val + self.next = next + + +class Solution: + def reorderList(self, head: ListNode | None) -> None: + if head is None: + return + + def find_middle(head: ListNode | None) -> ListNode | None: + # when the number of nodes is odd, the middle node is the left half of the list + # when the number of nodes is even, the middle node is the right half of the list + slow = head + fast = head + while fast and fast.next: + slow = slow.next + fast = fast.next.next + return slow + + middle = find_middle(head) + latter = middle.next + middle.next = None + + def reverse_list(head: ListNode | None) -> ListNode | None: + node = None + while head is not None: + next_head = head.next + head.next = node + node = head + head = next_head + return node + + reversed_latter = reverse_list(latter) + + def interleave(former: ListNode | None, latter: ListNode | None) -> None: + while former is not None and latter is not None: + next_former = former.next + next_latter = latter.next + former.next = latter + latter.next = next_former + former = next_former + latter = next_latter + + interleave(head, reversed_latter) diff --git a/0143.Reorder-List/step3.py b/0143.Reorder-List/step3.py new file mode 100644 index 0000000..94135a1 --- /dev/null +++ b/0143.Reorder-List/step3.py @@ -0,0 +1,45 @@ +# Definition for singly-linked list. +class ListNode: + def __init__(self, val=0, next=None): + self.val = val + self.next = next + + +class Solution: + def reorderList(self, head: ListNode | None) -> None: + if head is None: + return + + def find_middle(head): + slow = head + fast = head + while fast and fast.next: + slow = slow.next + fast = fast.next.next + return slow + + middle = find_middle(head) + latter = middle.next + middle.next = None + + def reverse_list(head): + node = None + while head is not None: + next_head = head.next + head.next = node + node = head + head = next_head + return node + + reversed_latter = reverse_list(latter) + + def merge_lists(former, latter): + while former is not None and latter is not None: + next_former = former.next + next_latter = latter.next + former.next = latter + latter.next = next_former + former = next_former + latter = next_latter + + merge_lists(head, reversed_latter) From abafa00008b24199cdc4be275bdb021cd30b2974 Mon Sep 17 00:00:00 2001 From: tom4649 Date: Sun, 28 Jun 2026 06:44:24 +0900 Subject: [PATCH 2/2] Add suggested changes --- 0143.Reorder-List/step1_two_pointer_revised.py | 2 +- 0143.Reorder-List/step2.py | 2 +- 0143.Reorder-List/step3.py | 2 +- 3 files changed, 3 insertions(+), 3 deletions(-) diff --git a/0143.Reorder-List/step1_two_pointer_revised.py b/0143.Reorder-List/step1_two_pointer_revised.py index 9df2ad9..5c34f70 100644 --- a/0143.Reorder-List/step1_two_pointer_revised.py +++ b/0143.Reorder-List/step1_two_pointer_revised.py @@ -10,7 +10,7 @@ def reorderList(self, head: Optional[ListNode]) -> None: slow = head fast = head - while fast and fast.next: + while fast is not None and fast.next is not None: slow = slow.next fast = fast.next.next diff --git a/0143.Reorder-List/step2.py b/0143.Reorder-List/step2.py index 4d86c8b..462051f 100644 --- a/0143.Reorder-List/step2.py +++ b/0143.Reorder-List/step2.py @@ -15,7 +15,7 @@ def find_middle(head: ListNode | None) -> ListNode | None: # when the number of nodes is even, the middle node is the right half of the list slow = head fast = head - while fast and fast.next: + while fast is not None and fast.next is not None: slow = slow.next fast = fast.next.next return slow diff --git a/0143.Reorder-List/step3.py b/0143.Reorder-List/step3.py index 94135a1..31655ba 100644 --- a/0143.Reorder-List/step3.py +++ b/0143.Reorder-List/step3.py @@ -13,7 +13,7 @@ def reorderList(self, head: ListNode | None) -> None: def find_middle(head): slow = head fast = head - while fast and fast.next: + while fast is not None and fast.next is not None: slow = slow.next fast = fast.next.next return slow