Update reverse and reverseK functions:

Fix q_reverse to correctly swap prev and next pointers while traversing
the queue. The previous implementation did not properly update the head
node's next and prev pointers, leading to potential list corruption.

Key fixes:
Ensure that head->next and head->prev are correctly updated.
Traverse the list safely without modifying pointers
before they are used.
Maintain circular doubly linked list structure.

Change-Id: I31bdc6ec4d67287c16852fefbdad13f389ef97fb

Author: I-Ying-Tsai

queue.c

@@ -95,6 +95,7 @@ element_t *q_remove_head(struct list_head *head, char *sp, size_t bufsize)
         strncpy(sp, node->value, bufsize - 1);
         sp[bufsize - 1] = '\0';
     }
+
     return node;
 }
 
@@ -114,6 +115,7 @@ element_t *q_remove_tail(struct list_head *head, char *sp, size_t bufsize)
         strncpy(sp, node->value, bufsize - 1);
         sp[bufsize - 1] = '\0';
     }
+
     return node;
 }
 
@@ -220,29 +222,34 @@ void q_reverse(struct list_head *head)
         return;
     }
 
-    struct list_head *cur = head->prev;
+    struct list_head *cur = head, *tmp;
 
-    while (cur != head) {
-        struct list_head *prev = cur->prev;
-        list_move(cur, head);
-        cur = prev;
-    }
+    do {
+        tmp = cur->next;
+        cur->next = cur->prev;
+        cur->prev = tmp;
+        cur = tmp;
+    } while (cur != head);
+    return;
 }
 
 /* Reverse the nodes of the list k at a time */
 void q_reverseK(struct list_head *head, int k)
 {
-    if (!head || list_empty(head) || k <= 1)
+    if (!head || list_empty(head) || k <= 1) {
         return;
+    }
 
     struct list_head *cur, *next;
     int count = 0;
 
-    list_for_each (cur, head)
+    list_for_each (cur, head) {
         count++;
+    }
 
-    if (count < k)
+    if (count < k) {
         return;
+    }
 
     cur = head->next;
 
@@ -255,9 +262,14 @@ void q_reverseK(struct list_head *head, int k)
                 break;
             list_move_tail(next, head);
         }
+
+        group_tail->prev = cur->prev;
+        cur->prev->next = group_tail;
+
         cur = group_tail->next;
         count -= k;
     }
+    return;
 }
 
 /* Sort elements of queue in ascending/descending order */