Implement q_reverse

Previously, q_reverse was an empty placeholder. This commit
implements the function to properly reverse the elements of
a queue using a safe traversal approach.

Change-Id: I7fd9c076d1b690134a553ac18238fe7bbdc3a3ba

Author: yy214123

queue.c

@@ -138,7 +138,20 @@ void q_swap(struct list_head *head)
 }
 
 /* Reverse elements in queue */
-void q_reverse(struct list_head *head) {}
+void q_reverse(struct list_head *head)
+{
+    if (!head || list_empty(head))
+        return;
+    struct list_head *curr, *safe, *temp;
+    list_for_each_safe (curr, safe, head) {
+        temp = curr->next;
+        curr->next = curr->prev;
+        curr->prev = temp;
+    }
+    temp = head->next;
+    head->next = head->prev;
+    head->prev = temp;
+}
 
 /* Reverse the nodes of the list k at a time */
 void q_reverseK(struct list_head *head, int k)