链表--合并K个排序链表

合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。
思路：分治思想。处理相加为len-1的数据。也就是0对应的len-i-1。类似归并排序思想。两两对应处理。

    private static class ListNode {
        private int val
        private ListNode next;
        public ListNode() {
        }
        public ListNode(int val) {
            this.val = val;
        }
        public ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }

public static ListNode mergeKLists(ListNode[] lists) {
        int len = lists.length;
        if (len == 0) {
            return null;
        }

        //使用分治法思想，两两合并。归并排序类似
        while (len > 1) {
            //分治处理，
            for (int i = 0; i < len / 2; i++) {
                lists[i] =mergeTowList(lists[i], lists[len - 1 - i]);
            }

            //处理后剩余数量
            len = (len + 1)/2;
        }

        return lists[0];
    }

    public static ListNode mergeTowList(ListNode first, ListNode second) {
        //设置一个哨兵服务结点
        ListNode head = new ListNode(-1);
        ListNode p = head;
        while (first != null && second != null) {
            if (first.val < second.val) {
                p.next = first;
                first = first.next;
            } else {
                p.next = second;
                second = second.next;
            }
            p = p.next;

        }

        if (first != null) {
            p.next = first;
        }

        if (second != null) {
            p.next = second;
        }

        //取下一个结点
        return head.next;
    }