LeetCode每日一题：merge k sorted list

问题描述

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

问题分析

我们可以使用以前写过的合并两条有序链表mergeTwoLists()函数，循环k次即可，复杂度是O(n*k)=O(n^2)。

代码实现

public ListNode mergeKLists(ArrayList<ListNode> 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;
    }