From 09645aaad3b35b70c51c28f529aec4aad5d0442d Mon Sep 17 00:00:00 2001 From: maeken4 Date: Sat, 31 May 2025 00:26:20 +0900 Subject: [PATCH 1/4] step1 --- 206.-Reverse-Linked-List/memo.md | 42 ++++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+) create mode 100644 206.-Reverse-Linked-List/memo.md diff --git a/206.-Reverse-Linked-List/memo.md b/206.-Reverse-Linked-List/memo.md new file mode 100644 index 0000000..2b1119b --- /dev/null +++ b/206.-Reverse-Linked-List/memo.md @@ -0,0 +1,42 @@ +# step1 +素直に書いてガチャガチャ変えて動いてしまった。よくない。 +```cpp:step1.cpp +#include + +class Solution { + public: + ListNode* reverseList(ListNode* head) { + std::stack st; + ListNode* tail = head; + // nullをはじくのを統一的に書けないか？ + if (head == nullptr) { + return nullptr; + } + while(tail != nullptr) { + st.push(tail); + tail = tail->next; + } + head = st.top(); + st.pop(); + tail = head; + while(!st.empty()) { + tail->next = st.top(); + st.pop(); + tail = tail->next; + } + // これってなんで必要？ + tail->next = nullptr; + return head; + } +}; + +``` + +# step2 +```cpp:step2.cpp +#include + +class Solution { + +} +``` From 9e75ee86808be5bdb206878cd84feac479d89aa6 Mon Sep 17 00:00:00 2001 From: maeken4 Date: Sat, 31 May 2025 17:27:59 +0900 Subject: [PATCH 2/4] step2 --- 206.-Reverse-Linked-List/memo.md | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) diff --git a/206.-Reverse-Linked-List/memo.md b/206.-Reverse-Linked-List/memo.md index 2b1119b..28a116c 100644 --- a/206.-Reverse-Linked-List/memo.md +++ b/206.-Reverse-Linked-List/memo.md @@ -33,10 +33,23 @@ class Solution { ``` # step2 -```cpp:step2.cpp -#include +```cpp:step2.cpp class Solution { - -} + public: + ListNode* reverseList(ListNode* head) { + return reverseRecursively(head); + } + private: + // node以降のリストを逆順に繋ぎなおし、元々の最後尾を新たなheadとして返す + ListNode* reverseRecursively(ListNode* node) { + if (node == nullptr || node->next == nullptr) { + return node; + } + ListNode* head = reverseRecursively(node->next); + node->next->next = node; + node->next = nullptr; + return head; + } +}; ``` From e30de22cd54e0c4dab8713af400534b11d6adb69 Mon Sep 17 00:00:00 2001 From: maeken4 Date: Sun, 1 Jun 2025 02:27:57 +0900 Subject: [PATCH 3/4] step3 --- 206.-Reverse-Linked-List/memo.md | 104 ++++++++++++++++++++++++++++++- 1 file changed, 102 insertions(+), 2 deletions(-) diff --git a/206.-Reverse-Linked-List/memo.md b/206.-Reverse-Linked-List/memo.md index 28a116c..b92f596 100644 --- a/206.-Reverse-Linked-List/memo.md +++ b/206.-Reverse-Linked-List/memo.md @@ -1,5 +1,6 @@ # step1 -素直に書いてガチャガチャ変えて動いてしまった。よくない。 +- スタックのセクションにあったのでとりあえずスタックで解いた。 +- 素直に書いてガチャガチャ変えて動いてしまった。よくない。 ```cpp:step1.cpp #include @@ -33,7 +34,7 @@ class Solution { ``` # step2 - +- 再帰で書けることに気づく。 ```cpp:step2.cpp class Solution { public: @@ -53,3 +54,102 @@ class Solution { } }; ``` +人のコードを見る。 +- step1ですっきりしなかった部分も番兵を使うときれいに書ける([linoahhhさん](https://github.com/lilnoahhh/leetcode/pull/10/files/31a6eddb4e971614db2b6d2fb67da1a44fb764c4#r1948006046))。 +- スタックに入っているノードを取り出してつなげる、というのに最後の場合分けが生じなくてよくなる。 +```cpp +#include + +class Solution { + public: + ListNode* reverseList(ListNode* head) { + std::stack st; + ListNode* sentinel = nullptr; + st.push(sentinel); + ListNode* tail = head; + while (tail) { + st.push(tail); + tail = tail->next; + } + + ListNode* new_head = st.top();st.pop(); + tail = new_head; + while (!st.empty()) { + tail->next = st.top(); + st.pop(); + tail = tail->next; + } + return head; + } +}; +``` + + +- もっと素直に前後を入れ替えていく([tarinaihitoriさん](https://discord.com/channels/1084280443945353267/1295357747545505833/1298524551592018003)) +```cpp:step2_another.cpp +class Solution { + public: + ListNode* reverseList(ListNode* head){ + // 反転済みリストの先頭 + ListNode* reversed_head = nullptr; + ListNode* cur_node = head; + while (cur_node) { + ListNode* tmp = cur_node->next; + tail->next = reversed_head; + reversed_head = cur_node; + cur_node = tmp; + } + return reversed_head; + } +}; +``` + +シンプルな問だけどけっこう方針が違う解法があって面白い。 + + +# step3 +スタック +```cpp:step3.cpp +#include +class Solution { +public: + ListNode* reverseList(ListNode* head) { + std::stack st; + auto sentinel = nullptr; + st.push(sentinel); + auto tail = head; + while (tail) { + st.push(tail); + tail = tail->next; + } + + auto new_head = st.top(); + st.pop(); + tail = new_head; + while (tail) { + tail->next = st.top(); + st.pop(); + tail = tail->next; + } + return new_head; + } +}; +``` + +シンプルな交換 +```cpp +class Solution { +public: + ListNode* reverseList(ListNode* head) { + ListNode* reversed_head = nullptr; + ListNode* cur_node = head; + while (cur_node) { + auto tmp = cur_node->next; + cur_node->next = reversed_head; + reversed_head = cur_node; + cur_node = tmp; + } + return reversed_head; + } +}; +``` From a5ec461fe8d9e7dffbc36178e2126dedb74d4986 Mon Sep 17 00:00:00 2001 From: maeken4 Date: Sun, 1 Jun 2025 19:51:00 +0900 Subject: [PATCH 4/4] add another solution --- 206.-Reverse-Linked-List/memo.md | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/206.-Reverse-Linked-List/memo.md b/206.-Reverse-Linked-List/memo.md index b92f596..aa81b80 100644 --- a/206.-Reverse-Linked-List/memo.md +++ b/206.-Reverse-Linked-List/memo.md @@ -153,3 +153,25 @@ public: } }; ``` + +再帰別解(https://github.com/maeken4/Arai60/pull/7/files/e30de22cd54e0c4dab8713af400534b11d6adb69#r2118170691) +```cpp +class Solution { + public: + ListNode* reverseList(ListNode* head) { + return reverse_and_append(head, nullptr); + } + private: + // [1,2,3], [4,5,6] -> [3,2,1,4,5,6] + // [], [4,5,6] -> [4,5,6s] + ListNode* reverse_and_append(ListNode* head1, ListNode* head2){ + if (head1 == nullptr) { + return head2; + } + auto new_head1 = head1->next; + head1->next = head2; + return reverse_and_append(new_head1, head1); + + } +}; +```