/*
 * @lc app=leetcode id=23 lang=java
 *
 * [23] Merge k Sorted Lists
 */

// @lc code=start
/**
 * 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 == null || lists.length == 0) {
            return null;
        }
        return mergeRecursively(lists, 0, lists.length-1);
    }

    private ListNode mergeRecursively(ListNode[] lists, int i, int j) {
        if (i > j) {
            return null;
        }else if(i == j) {
            return lists[i];
        } else if (j - i == 1) {
            return merge(lists[i], lists[j]);
        }
        int m = (i+j)/2;
        return merge(mergeRecursively(lists, i, m), mergeRecursively(lists, m+1, j));
    }

    private ListNode merge(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0), tail = dummy;
        dummy.next = null;
        while(l1 != null || l2 != null) {
            if(l1 == null || l2 == null) {
                if(l1 != null) {
                    tail.next = l1;
                } else if(l2 != null) {
                    tail.next = l2;
                }
                break;
            }
            if(l1.val <= l2.val) {
                tail.next = l1;
                l1 = l1.next;
                tail = tail.next;
                tail.next = null;
            } else {
                tail.next = l2;
                l2 = l2.next;
                tail = tail.next;
                tail.next = null;
            }
        }
        return dummy.next;
    }
}
// @lc code=end