Leetcode 23. Merge K sorted lists

Solution.java

import java.util.PriorityQueue;

class Solution {

        class ListNodeWrapper implements Comparable<ListNodeWrapper> {
            int listNumber;
            int val;

            ListNodeWrapper(int listNumber, ListNode listNode) {
                this.listNumber = listNumber;
                this.val = listNode.val;
            }

            @Override
            public int compareTo(ListNodeWrapper listNodeWrapper) {
                return this.val - listNodeWrapper.val;
            }
        }

        public ListNode mergeKLists(ListNode[] lists) {
            PriorityQueue<ListNodeWrapper> nextValues = initQueue(lists);
            ListNode result = null;
            ListNode ptr = null;
            while(!nextValues.isEmpty()) {
                ListNodeWrapper ln = nextValues.poll();
                if(result == null) {
                    result = lists[ln.listNumber];
                    ptr = result;
                } else {
                    ptr.next = lists[ln.listNumber];
                    ptr = ptr.next;
                }
                if(lists[ln.listNumber] != null) {
                    lists[ln.listNumber] = lists[ln.listNumber].next;
                }
                if(lists[ln.listNumber] != null) {
                    nextValues.add(new ListNodeWrapper(ln.listNumber, lists[ln.listNumber]));
                }
            }
            return result;
        }

        private PriorityQueue<ListNodeWrapper> initQueue(ListNode[] lists) {
            PriorityQueue<ListNodeWrapper> nextValues = new PriorityQueue<>();
            for(int x = 0; x < lists.length; x++) {
                if(lists[x] == null) {
                    continue;
                }
                ListNodeWrapper ln = new ListNodeWrapper(x, lists[x]);
                nextValues.add(ln);
            }
            return nextValues;
        }
    }