Implement q_delete_mid

Previously, q_delete_mid simply returned true without performing
any deletion. This change implements the actual logic to find
and remove the middle node from a doubly circular linked list.

Instead of using the traditional slow-fast pointer approach,
this implementation adopts a two-pointer convergence strategy,
where pointers move inward from both ends simultaneously.
This reduces the number of steps compared to the slow-fast
pointer method, improving efficiency.

Specifically, for a list of size n, this approach decreases
the number of traversals by approximately n/2 steps.

Change-Id: Ie9d2eadd23600f2999267151ed5c9468ffebf208

Author: yy214123

queue.c

@@ -111,6 +111,16 @@ int q_size(struct list_head *head)
 bool q_delete_mid(struct list_head *head)
 {
     // https://leetcode.com/problems/delete-the-middle-node-of-a-linked-list/
+    if (!head || list_empty(head))
+        return false;
+    struct list_head *access_next = head->next;
+    struct list_head *access_prev = head->prev;
+    while ((access_next != access_prev) && (access_prev->next != access_next)) {
+        access_next = access_next->next;
+        access_prev = access_prev->prev;
+    }
+    list_del(access_prev);
+    q_release_element(list_entry(access_prev, element_t, list));
     return true;
 }