Implement q_descend using q_ascend and reversal

Previously, q_descend was a placeholder and did nothing.
Now it correctly removes nodes that violate descending order
by reusing q_ascend with two queue reversals.

This approach leverages the logic of q_ascend logic first reversing
the queue, applying q_ascend to remove non-decreasing elements,
and then reversing it back to restore the original order.

Change-Id: I00e4450e1e2605c204b4ab2cbba9fbb905f43e3d

Author: yy214123

queue.c

@@ -219,7 +219,10 @@ int q_ascend(struct list_head *head)
 int q_descend(struct list_head *head)
 {
     // https://leetcode.com/problems/remove-nodes-from-linked-list/
-    return 0;
+    q_reverse(head);
+    q_ascend(head);
+    q_reverse(head);
+    return q_size(head);
 }
 
 /* Merge all the queues into one sorted queue, which is in ascending/descending