023 Merge k Sorted Lists 合并K个有序链表

合并K个有序链表，并且作为一个有序链表的形式返回。分析并描述它的复杂度。

详见：https://leetcode.com/problems/merge-k-sorted-lists/description/

实现语言：Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        int n=lists.length;
        if(n==0){
            return null;
        }
        while(n>1){
            int k=(n+1)/2;
            for(int i=0;i<n/2;++i){
                lists[i]=mergeTwoLists(lists[i],lists[i+k]);
            }
            n=k;
        }
        return lists[0];
    }
    private ListNode mergeTwoLists(ListNode l1,ListNode l2){
        if(l1==null&&l2==null){
            return null;
        }
        if(l1==null){
            return l2;
        }
        if(l2==null){
            return l1;
        }
        ListNode head=new ListNode(-1);
        ListNode cur=head;
        while(l1!=null&&l2!=null){
            if(l1.val<l2.val){
                cur.next=l1;
                l1=l1.next;
            }else{
                cur.next=l2;
                l2=l2.next;
            }
            cur=cur.next;
        }
        cur.next=l1!=null?l1:l2;
        return head.next;
    }
}

参考：https://www.cnblogs.com/grandyang/p/4606710.html