Implement q_swap function

Perform the operation of swapping pairs of entries in the queue, without swapping if there is only one entry left at the end.

queue.c

@@ -137,7 +137,37 @@ bool q_delete_dup(struct list_head *head)
 /* Swap every two adjacent nodes */
 void q_swap(struct list_head *head)
 {
- // https://leetcode.com/problems/swap-nodes-in-pairs/
+ // https://leetcode.com/problems/swap-nodes-in-pairs//
+ struct list_head *cur = head;
+ struct list_head *first = head->next;
+ struct list_head *second = first->next;
+
+ while (first != head) {
+ if (second == head) {
+ cur->next = first;
+ first->prev = cur;
+
+ first->next = head;
+ head->prev = first;
+
+ break;
+ } else {
+ first->next = second->next;
+ second->next->prev = first;
+
+ second->next = first;
+ first->prev = second;
+
+ cur->next = second;
+ second->prev = cur;
+
+ cur = first;
+ first = first->next;
+ second = first->next;
+ }
+ cur->next = head;
+ head->prev = cur;
+ }
 }
 
 /* Reverse elements in queue */