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
        if (lists.size() == 0) {
            return null;
        }
        return mergeSort(lists, 0, lists.size() - 1);
    }

    public ListNode mergeSort(ArrayList<ListNode> lists, int l, int r) {
        if (l == r) {
            return lists.get(l);
        }
        int mid = (l + r) >> 1;
        return sort(mergeSort(lists, l, mid), mergeSort(lists, mid + 1, r));
    }

    public ListNode sort(ListNode p1, ListNode p2) {
        ListNode h = new ListNode(0);
        ListNode cur = h;
        while (p1 != null && p2 != null) {
            if (p1.val < p2.val) {
                cur.next = p1;
                p1 = p1.next;
            } else {
                cur.next = p2;
                p2 = p2.next;
            }

            cur = cur.next;
        }
        if (p1 != null) {
            cur.next = p1;
        }

        if (p2 != null) {
            cur.next = p2;
        }
        return h.next;
    }
}

使用归并排序的思想