题解 | #合并k个已排序的链表#

import java.util.*;
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ArrayList<ListNode> lists) {
        if(lists.size() == 0){
            return null;
        }

        return mergeRange(lists, 0, lists.size()-1);
        
    }

    private static ListNode mergeRange(ArrayList<ListNode> lists, int start, int end){
        //起始和结束位置相同，直接返回。
        if(start == end){
            return lists.get(start);
        }

        int mid = (start + end) / 2;

        return merge(mergeRange(lists, start, mid), mergeRange(lists, mid + 1, end));
    }

    private static ListNode merge(ListNode list1, ListNode list2){
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        while(list1 != null && list2 != null){
            if(list1.val < list2.val){
                cur.next = list1;
                cur = cur.next;
                list1 = list1.next;
            }else{
                cur.next = list2;
                cur = cur.next;
                list2 = list2.next;
            }
        }

        if(list1 != null){
            cur.next = list1;
        }

        if(list2 != null){
            cur.next = list2;
        }

        return dummy.next;
    }
}