leetCode 23. Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6

采用分治算法，时间复杂度为nlog(n)，同分治排序。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    //返回lists数组第low至high位置(不包括high位置)的链表merge后的链表
    public ListNode result(ListNode[] lists,int low,int high){
        if (low+1==high) return lists[low];
        int mid=(low+high)/2;
        ListNode left = result(lists,low,mid);
        ListNode right = result(lists,mid,high);

        //merge left链表和right链表
        ListNode head = new ListNode(-65535);
        ListNode p = head;//指向head最后一个节点
        while (left!=null && right!=null){
            if (left.val < right.val){
                p.next=left;
                left=left.next;
            }else {
                p.next=right;
                right=right.next;
            }
            p=p.next;
        }
        if (left!=null){
            p.next=left;
        }
        if (right!=null){
            p.next=right;
        }
        return head.next;
    }

    public ListNode mergeKLists(ListNode[] lists) {
        if (lists.length==0) return null;
        if (lists.length==1) return lists[0];
        ListNode result = result(lists, 0, lists.length);
        return result;
    }
}