/**
 * 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.length == 0)
            return null;
        return split(lists, 0, lists.length - 1);
    }

    public ListNode split(ListNode[] lists, int i, int j) {
        if (i == j)
            return lists[i];
        int m = (i + j) >>> 1;
        ListNode left = split(lists, i, m);
        ListNode right = split(lists, m + 1, j);
        return mergeTwoLists(left, right);
    }

    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode(0);
        ListNode current = dummy;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                current.next = list1;
                list1 = list1.next;
            } else {
                current.next = list2;
                list2 = list2.next;
            }
            current = current.next;
        }
        if (list1 == null) {
            current.next = list2;
            return dummy.next;
        }
        if (list2 == null) {
            current.next = list1;
            return dummy.next;
        }
        return dummy.next;
    }
}