add reverse k, merge sort

Author: Alan Tseng

docs/BMC/openbmc.md

@@ -0,0 +1 @@
+[The overview and future of Open Source FW for server industry](https://youtu.be/cnC4zmxm5AY?si=c0G3e_4zOFUZe7g2)

docs/DSA/Linked_List/0.linked_list.md

@@ -293,6 +293,89 @@ void reverse(Node *head)
     head->next = prev;
 }
 ```
+
+### 實作反轉 K 個節點
+![alt text](assets/image-12.png)
+```c
+/* 以 dummy 為鏈首，將串列每 k 個節點反轉 */
+void reverseK(Node *dummy, int k) {
+    puts("reverseK");
+    if (k <= 1 || !dummy || !dummy->next) return;
+
+    Node *group_prev = dummy;          /* 目前這組前一個節點（首輪為 dummy） */
+
+    while (1) {
+        /* 1️⃣ 找到這組第 k 個節點 (kth) */
+        Node *kth = group_prev;
+        for (int i = 0; i < k && kth; ++i)
+            kth = kth->next;
+        if (!kth) break;               /* 剩餘不足 k 個，不反轉 */
+
+        Node *group_next = kth->next;  /* 下一組開頭 */
+        /* 2️⃣ 就地反轉 [group_prev->next, kth] 之間的指標 */
+        Node *prev = group_next;
+        Node *curr = group_prev->next;
+        while (curr != group_next) {
+            Node *tmp = curr->next;
+            curr->next = prev;
+            prev       = curr;
+            curr       = tmp;
+        }
+        /* 3️⃣ 接回前後串列 */
+        Node *old_group_head = group_prev->next;
+        group_prev->next = kth;
+        group_prev = old_group_head;   /* 為下一輪準備：此時它是反轉後的尾 */
+    }
+}
+```
+### 實作 merge sort linked list
+![alt text](assets/image-13.png)
+```c
+// Merge two sorted lists using comparator
+Node* merge(Node *a, Node *b, int (*cmp)(int, int)) {
+    Node dummy;
+    Node *tail = &dummy;
+    dummy.next = NULL;
+    while (a && b) {
+        if (cmp(a->data, b->data) <= 0) {
+            tail->next = a; // 接上較小的節點
+            a = a->next;
+        } else {
+            tail->next = b;
+            b = b->next;
+        }
+        tail = tail->next;
+    }
+    tail->next = a ? a : b; // 接上剩餘的節點
+    return dummy.next;
+}
+
+// Helper: merge sort for linked list with comparator
+static Node* merge_sort_internal(Node* head, int (*cmp)(int, int)) {
+    if (!head || !head->next) return head;
+    Node *slow = head, *fast = head->next;
+    while (fast && fast->next) {
+        slow = slow->next;
+        fast = fast->next->next;
+    }
+    Node *mid = slow->next;
+    slow->next = NULL;
+    Node *left = merge_sort_internal(head, cmp);
+    Node *right = merge_sort_internal(mid, cmp);
+    return merge(left, right, cmp);
+}
+
+// Sort using merge sort with comparator function
+void sort(Node *node, int (*cmp)(int, int)) {
+    node->next = merge_sort_internal(node->next, cmp);
+}
+// Convenience: ascending order comparator
+int cmp_asc(int a, int b) { return a - b; }
+// Convenience: descending order comparator
+int cmp_desc(int a, int b) { return b - a; }
+```
+
+
 ## 編譯
 ```
 gcc -Wall -o linked_list.out main.c list.c 

docs/DSA/Linked_List/assets/image-12.png

@@ -0,0 +1,3 @@
+
+
+https://hackmd.io/@sysprog/info2024/https%3A%2F%2Fhackmd.io%2F%40sysprog%2FH1-4KbanC

docs/DSA/Linked_List/assets/image-13.png

@@ -0,0 +1,13 @@
+
+- `git diff`
+    - `-stats`: show stats of changes
+    - `--cached`: show staged changes
+
+
+- `git reset ~head~1`
+
+- `git commit`
+    `-s`: sign off on the commit
+    `-m`: commit message
+    `--amend`: amend the last commit
+