题解 | #合并k个已排序的链表#

1. 合并两个升序链表
2. 使用递归，多路归并

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 == null || lists.size() == 0) {
            return null;
        } else {
            return mergeKLists(lists, 0, lists.size() - 1);
        }
    }
    public ListNode mergeKLists (ArrayList<ListNode> lists, int left, int right) {
        // write code here
        if (left == right) {
            return lists.get(left);
        }
        int mid = (left + right) / 2;
        return Merge(mergeKLists(lists, left, mid), mergeKLists(lists, mid + 1, right));
    }

    public ListNode Merge (ListNode pHead1, ListNode pHead2) {
        // write code here
        ListNode dummynode  = new ListNode(-1);

        dummynode.next = pHead1;
        ListNode pre = dummynode;
        while (pHead1 != null && pHead2 != null) {
            if (pHead2.val < pHead1.val ) {
                ListNode next = pHead2.next;

                pHead2.next = pHead1;
                pre.next = pHead2;
                pHead2 = next;
                pre = pre.next;
            } else {
                pre = pHead1;
                pHead1 = pHead1.next;
            }
        }

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

        return dummynode.next;


    }
}