Merge k Sorted Lists

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null) {
            throw new IllegalArgumentException("aa");
        }
        if (lists.length == 0) {
            return null;
        }
        return helper(lists, 0, lists.length - 1);
    }
    private ListNode helper(ListNode[] lists, int start, int end) {
        if (start == end) {
            return lists[start];
        }
        int mid = start + (end - start) / 2;
        ListNode left = helper(lists, start, mid);
        ListNode right = helper(lists, mid + 1, end);
        ListNode node = merge(left, right);
        return node;
    }
    private ListNode merge(ListNode left, ListNode right) {
        ListNode dummy = new ListNode(0);
        ListNode node = dummy;
        while (left != null && right != null) {
            if (left.val < right.val) {
                node.next = left;
                left = left.next;
            } else {
                node.next = right;
                right = right.next;
            }
            node = node.next;
        } 
        if (left != null) {
            node.next = left;
        }
        if (right != null) {
            node.next = right;
        }
        return dummy.next;
    }
}