题解 | #合并k个已排序的链表#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param lists ListNode类ArrayList
     * @return ListNode类
     */
    public ListNode mergeKLists (ArrayList<ListNode> lists) {
        // write code here
        int num = lists.size();
        if(num == 0) return null;

        while(num > 1){
            for(int i = 0; i < num/2; i++){
                lists.set(i, ListMerge(lists.get(i*2), lists.get(i*2+1)));
            }

            if(num % 2 != 0){
                lists.set(num/2, lists.get(num - 1));
            }

            num = num - num/2;
        }
        return lists.get(0);
    }
    public ListNode ListMerge(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;

        ListNode head = new ListNode(0);
        ListNode res = head;

        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                head.next = l1;
                l1 = l1.next;
            } else {
                head.next = l2;
                l2 = l2.next;
            }
            head = head.next;
        }

        if(l1 != null)
            head.next = l1;
        else
            head.next = l2;

        return res.next;
    }
}