23. Merge k Sorted Lists /Leetcode

MergekSortedLists.java

public class MergekSortedLists {
    public static void main(String[] args) {

        ListNode listNode = new ListNode(1);
        listNode.next = new ListNode(4);
        listNode.next.next = new ListNode(5);

        ListNode secondListNode = new ListNode(1);
        secondListNode.next = new ListNode(3);
        secondListNode.next.next = new ListNode(4);

        ListNode thirdListNode = new ListNode(2);
        thirdListNode.next = new ListNode(6);
        ListNode[] lists = {listNode, secondListNode, thirdListNode};

        displayNodes(mergeKLists(lists));
    }

    public static ListNode mergeKLists(ListNode[] lists) {
        PriorityQueue<ListNode> nodes = new PriorityQueue<>((o1, o2) -> o1.val - o2.val);
        for (ListNode list : lists) {
            ListNode current = list;
            while (current != null) {
                //All new Nodes Have a null next element so when building final linkedlist attribute the correct next element
                nodes.add(new ListNode(current.val));
                current = current.next;
            }
        }
        
        ListNode root = nodes.poll();
        ListNode current = root;
        while (!nodes.isEmpty()) {
            current.next = nodes.poll();
            if (nodes.peek() != null) {
                current = current.next;
            }
        }
        
        return root;
    }

    public static void displayNodes(ListNode listNode) {
        ListNode current = listNode;
        while (current != null) {
            System.out.print(" " + current.val + " ");
            current = current.next;
        }
    }
    public static class ListNode {
        public int val;
        public ListNode next;
        
        public ListNode() {
        }

        public ListNode(int x) {
            val = x;
        }
    }
}