based on Merge 2 sorted lists.

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

可以由k = (n+1)/2和后面的n = k看出这种辗转对半缩短的方法, (n+1)/2这种选择是非常重要的; (n+1)/2这种, n增加1之后会确实进1, i.e. n=0时对应的链表是lists[0]和lists[n/2],n=1时是lists[1]和lists[n/2 + 1], 这样合并的链表就确实成对地变化; 如果时n/2就不行

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class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists.length == 0) return null;
        int n = lists.length;
        int k = 0;
        while (n > 1) {
            k = (n + 1) / 2;
            for (int i = 0; i < n / 2; ++i) {
                lists[i] = mergeTwoLists(lists[i], lists[i + k]);
            }
            n= k;
        }
        return lists[0];
    }
    
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0), cur = dummy;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                cur.next = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        cur.next = l1 != null ? l1 : l2;
        return dummy.next;
    }
}