【hot100-java】合并 K 个升序链表

链表篇

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    //先实现合并两个有序链表
    public ListNode mergeTwoLists(ListNode list1,ListNode list2){
        ListNode cur=new ListNode(0);
        //哨兵节点
        ListNode dummy=cur;
        while(list1!=null&&list2!=null){
            if(list1.val<list2.val){
                cur.next=list1;
                list1=list1.next;
            }else{
                cur.next=list2;
                list2=list2.next;
            }
            cur=cur.next;
        }
        //拼接剩余链表
        cur.next=list1!=null?list1:list2;
        return dummy.next;
    }
    
    //合并所有升序链表
    public ListNode mergeKLists(ListNode[] lists) {
       int m=lists.length;
       if(m==0){
        return null;
       }
       if(m==1){
        return lists[0];
       }
       //二分法，切成两半
       int mid=m/2;
       ListNode left=mergeKLists(java.util.Arrays.copyOfRange(lists,0,mid));
       ListNode right=mergeKLists(java.util.Arrays.copyOfRange(lists,mid,m));
       return mergeTwoLists(left,right);
    }
}