LeetCode毎日1題:merge k sorted list

問題の説明
Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.
もんだいぶんせき
以前に書いた2つの秩序チェーンテーブルmergeTwoLists()関数を用いて,k回ループすればよいが,複雑度はO(n*k)=O(n^2)である.
コード実装

public ListNode mergeKLists(ArrayList lists) {
        if (lists.size() == 0) return null;
        ListNode head = lists.get(0);
        for (int i = 1; i < lists.size(); i++) {
            head = mergeTwoLists(head, lists.get(i));
        }
        return head;
    }
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode cur = head;
        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;
        }
        if (l1 == null) {
            cur.next = l2;
        }
        if (l2 == null) {
            cur.next = l1;
        }
        return head.next;
    }