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
        // 解题思路合并多个升序链表和合并2个升序链表思路一致，就是多了一层循环

        if (lists == null || lists.size() == 0) {
            return null;
        }

        if (lists.size() == 1) {
            return lists.get(0);
        }

        ListNode  n1 = lists.get(0);
        for (int i = 1; i < lists.size(); i++) {
           n1= merge(n1,lists.get(i));
        }
        return n1;
    }


    private ListNode merge(ListNode pHead1, ListNode pHead2) {
        ListNode cur = new ListNode(-1);
        ListNode root = cur;

        while (pHead1 != null || pHead2 != null) {
            if (pHead1 == null) {
                cur.next = pHead2;
                break;
            }

            if (pHead2 == null) {
                cur.next = pHead1;
                break;
            }

            int val1 = pHead1.val;
            int val2 = pHead2.val;

            if(val1 >val2){
                cur.next = pHead2;
                pHead2 = pHead2.next;
            }else{
                cur.next = pHead1;
                pHead1 = pHead1.next;
            }

            cur = cur.next;
        }

        return root.next;
    }
}