23. 合并 K 个升序链表

给你一个链表数组，每个链表都已经按升序排列。

请你将所有链表合并到一个升序链表中，返回合并后的链表。

示例 1：

输入：lists = [[1,4,5],[1,3,4],[2,6]]
输出：[1,1,2,3,4,4,5,6]
解释：链表数组如下：
[
1->4->5,
1->3->4,
2->6
]
将它们合并到一个有序链表中得到。
1->1->2->3->4->4->5->6
示例 2：

输入：lists = []
输出：[]
示例 3：

输入：lists = [[]]
输出：[]

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方法一：按照顺序，逐次合并，每次合并两个

/**
 * 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) {
        ListNode result = null;
        for (ListNode list : lists) {
            result = mergeTwoList(result, list);
        }
        return result;
    }

    public ListNode mergeTwoList(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        ListNode node1 = list1;
        ListNode node2 = list2;
        while (node1 != null && node2 != null) {
            if (node1.val <= node2.val) {
                tail.next = node1;
                node1 = node1.next;
            } else {
                tail.next = node2;
                node2 = node2.next;
            }
            tail = tail.next;
        }
        if (node1 != null) {
            tail.next = node1;
        } else if (node2 != null) {
            tail.next = node2;
        }
        return dummy.next;
    }
}

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方法二：分而治之

/**
 * 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) {
        return merge(lists, 0, lists.length - 1);
    }

    public ListNode merge(ListNode[] lists, int l, int r) {
        if (l == r) return lists[l];
        if (l > r) {
            return null;
        }
        int mid = (l + r) / 2;
        return mergeTwoList(merge(lists, l, mid), merge(lists, mid + 1, r));
    }

    public ListNode mergeTwoList(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        ListNode node1 = list1;
        ListNode node2 = list2;
        while (node1 != null && node2 != null) {
            if (node1.val <= node2.val) {
                tail.next = node1;
                node1 = node1.next;
            } else {
                tail.next = node2;
                node2 = node2.next;
            }
            tail = tail.next;
        }
        if (node1 != null) {
            tail.next = node1;
        } else if (node2 != null) {
            tail.next = node2;
        }
        return dummy.next;
    }
}