leetcode23.合并k个升序链表

套用归并排序的思想，要排序这k个链表，先排序好前k/2个链表，再排序好后k/2个链表，最后将这两个链表用普通的两链表合并就能解决问题 

/**
 * 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 {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists.length==0)
            return null;
        return mergeKLists(lists, 0, lists.length - 1);
    }
    
    private ListNode mergeKLists(ListNode[] lists, int left, int right) {
        if (left == right) {
            return lists[left];
        } else {
            int mid = (right - left) / 2 + left;
            ListNode list1 = mergeKLists(lists, left, mid);
            ListNode list2 = mergeKLists(lists, mid + 1, right);
            return merge2Lists(list1, list2);
        }
    }

    private ListNode merge2Lists(ListNode list1, ListNode list2) {
        ListNode dymmyHead = new ListNode(0, null);
        ListNode cur=dymmyHead;
        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;
        }
        while (list1 != null) {
            cur.next=list1;
            list1=list1.next;
            cur=cur.next;
        }
        while (list2 != null) {
            cur.next=list2;
            list2 = list2.next;
            cur = cur.next;
        }
        return dymmyHead.next;
    }
}