23.java

/**
 * 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;

        PriorityQueue<ListNode> heap =
            new PriorityQueue<>((first, second) -> first.val - second.val);

        for (ListNode node : lists) {
            if (node != null) {
                heap.offer(node);
            }
        }

        ListNode dummy = new ListNode(0);
        ListNode current = dummy;

        while (!heap.isEmpty()) {
            ListNode smallest = heap.poll();
            current.next = smallest;
            current = current.next;

            if (smallest.next != null) {
                heap.offer(smallest.next);
            }
        }

        return dummy.next;
    }
}