Number.23——合併K個排序鏈表

題目鏈接：https://leetcode-cn.com/problems/merge-k-sorted-lists/

合併 k 個排序鏈表，返回合併後的排序鏈表。請分析和描述算法的複雜度。

示例：

輸入:
[
1->4->5,
1->3->4,
2->6
]
輸出: 1->1->2->3->4->4->5->6

分而治之：很容易想到歸併排序，與歸併排序不同的是在第三個函數mergeList，這個我有點菜，理解不了爲什麼這樣寫 …

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) return null;
        return merge(lists, 0, lists.length - 1);
    }

    private ListNode merge(ListNode[] lists, int left, int right){
        if (left == right) return lists[left];
        int mid = left + (right - left) / 2;
        ListNode l1 = merge(lists, left, mid);
        ListNode l2 = merge(lists, mid + 1, right);
        return mergeList(l1, l2);
    }

    private ListNode mergeList(ListNode l1, ListNode l2){
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val < l2.val){
            l1.next = mergeList(l1.next, l2);
            return l1;
        } else{
            l2.next = mergeList(l2.next, l1);
            return l2;
        }
    }
}