23. Merge k Sorted Lists

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0) return null;
        ListNode head = new ListNode();
        ListNode tail = head;
        PriorityQueue<ListNode> pq = new PriorityQueue<>((o1, o2) -> Integer.compare(o1.val, o2.val));
        
        for(ListNode node : lists) {
            if(node!=null) {
                pq.add(node);
            }
        }
        
        while (!pq.isEmpty()) {
            tail.next = pq.poll();
            tail = tail.next;
            if (tail.next != null) {
                pq.add(tail.next);
            }
        }
        
        return head.next;
    }
}