[数据结构与算法]NC51 合并k个已排序的链表

NC51 合并k个已排序的链表

描述
合并\ k k 个已排序的链表并将其作为一个已排序的链表返回。分析并描述其复杂度。
示例1
输入：
[{1,2,3},{4,5,6,7}]
复制
返回值：
{1,2,3,4,5,6,7}

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 mergeKLists(ArrayList<ListNode> lists) {
        if(lists == null){
            return null ;
        }
        return mergelist(lists , 0 , lists.size() - 1) ;
    }
    public ListNode mergelist(ArrayList<ListNode> lists , int left , int right){
        if(left == right){
            return lists.get(left) ;
        }
        if(left > right){
            return null ;
        }
        int mid = left + (right - left) / 2 ;
        return merge(mergelist(lists , left , mid) , mergelist(lists , mid+1 , right) ) ;
    }
    public ListNode merge(ListNode l1 , ListNode l2){
        if(l1 == null || l2 == null){
            return l1 == null ? l2 : l1 ;
        }
        ListNode dummy = new ListNode(-1) ;
        ListNode curt = dummy ;
        while(l1 != null && l2 != null){
            if(l1.val <= l2.val){
                curt.next = l1 ;
                l1 = l1.next ;
            }else{
                curt.next = l2 ;
                l2 = l2.next ;
            }
            curt = curt.next ;
        }
        if(l1 != null){
            curt.next = l1 ;
        }
        if(l2 != null){
            curt.next = l2 ;
        }
        return dummy.next ;
        
    }
}