Merge k Sorted Lists

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

Solution:

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode result=head;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                head.next = l1;
                l1 = l1.next;
            } else {
                head.next = l2;                
                l2 = l2.next;
            }
            head = head.next;
        }
        if(l1!=null){
            head.next=l1;
        }
        if(l2!=null){
            head.next=l2;
        } 
        return result.next;
    }
    
    public ListNode mergeKLists(ListNode[] lists) {        
        if(lists.length==0){
            return null;
        }
        if(lists.length==1){
            return lists[0];
        }
        ListNode head = mergeTwoLists(lists[0], lists[1]);
        for (int i = 2; i < lists.length; i++) {
            head = mergeTwoLists(head, lists[i]);
        }
        return head;
    }