LeetCode - Merge k Sorted Lists (Java)

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.
The simplest solution is using PriorityQueue. The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time (
public ListNode mergeKLists(ArrayList<ListNode> lists) {
if (lists.size() == 0)
return null;
//PriorityQueue is a sorted queue
PriorityQueue<ListNode> q = new PriorityQueue<ListNode>(lists.size(),
new Comparator<ListNode>() {
public int compare(ListNode a, ListNode b) {
if (a.val > b.val)
return 1;
else if(a.val == b.val)
return 0;
else 
return -1;
}
});
//add first node of each list to the queue
for (ListNode list : lists) {
if (list != null)
q.add(list);
}

ListNode head = new ListNode(0);
ListNode prev = head;

while (q.size() > 0) {
ListNode temp = q.poll();
prev.next = temp;

//keep adding next element of each list
if (temp.next != null)
q.add(temp.next);

prev = prev.next;
}

return head.next;

}
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