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 merge(ListNode a, ListNode b) {
        ListNode ans = new ListNode(0);
        ListNode tmp = ans;

        while (a != null && b != null) {
            if (a.val < b.val) {
                tmp.next = a;
                a = a.next;
            } else {
                tmp.next = b;
                b = b.next;
            }
            tmp = tmp.next;
        }
        if (a != null) {
            tmp.next = a;
        } else {
            tmp.next = b;
        }

        return ans.next;
    }

    public ListNode mergeKLists(ArrayList<ListNode> lists) {
        if(lists.isEmpty()){
            return null;
        }
        ListNode ans = lists.get(0);

        for (int i = 1; i < lists.size(); i++) {
            ans = merge(ans, lists.get(i));

        }

        return ans;
    }
}