合并 k 个有序链表

问题描述

* leetcode 23，  Merge k Sorted Lists

解题思路

1. 合并2个有序链表的进阶版，很容易想到，利用分治进行处理。

   把个数大于2的集合 两两划分处理，最后再做最后一步的处理。

   public class Solution {
       public ListNode mergeKLists(ListNode[] lists) {
           if (lists.length==0 || lists==null) { return null; }
           return merge(lists, 0, lists.length-1);
       }
       public static ListNode merge(ListNode[] lists, int left, int right) {
           if (left == right) { return lists[left]; }
           int mid = left + (right-left)/2;
           ListNode l1 = merge(lists, left, mid);
           ListNode l2 = merge(lists, mid+1, right);
           return merger2Lists(l1, l2);
       }
       public static ListNode merger2Lists(ListNode l1, ListNode l2) {
           // 递归写法
           if (l1==null) { return l2; }
           if (l2==null) { return l1; }
           ListNode head = null;
           if (l1.val < l2.val) {
               head = l1;
               head.next = merger2Lists(l1.next, l2);
           } else {
               head = l2;
               head.next = merger2Lists(l1, l2.next);
           }
           return head;
       }
   }