/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {

        if (lists == null || lists.length == 0) return null;

        int left = 0, right = lists.length - 1;                
        return divide(lists, left, right);    
    }

    private ListNode divide(ListNode[] lists, int left, int right){
        if (left == right){
            return lists[left];
        }

        int mid = left + (right - left) / 2;
        ListNode leftNode = divide(lists, left, mid);
        ListNode rightNode = divide(lists, mid + 1, right);

        return combine(leftNode, rightNode);
    }

    private ListNode combine(ListNode l1, ListNode l2){

        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        while (l1 != null && l2 != null){
            if (l1.val < l2.val){

                cur.next = l1;    
                cur = cur.next;
                l1 = l1.next;
            }
            else{

                cur.next = l2;    
                cur = cur.next;
                l2 = l2.next;
            }
        }

        if (l1 != null) cur.next = l1;
        if (l2 != null) cur.next = l2;

        return dummy.next;
    }
}