23. 合并K个排序链表

23. 合并K个排序链表

合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。

示例:

输入:
[
  1->4->5,
  1->3->4,
  2->6
]
输出: 1->1->2->3->4->4->5->6

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0){
            return null;
        }
        //每次合并两条
        int len = lists.length;
        while(len != 1){
            int i;
            for(i = 0;i < len / 2;i++){
                lists[i] = merge2(lists[2 * i],lists[2 * i + 1]);
            }
            if(len % 2 == 1){
                lists[i] = lists[len - 1];
                len++;
            }
            len /= 2;
        }
        return lists[0];
    }
    private ListNode merge2(ListNode l1,ListNode l2){
        if(l1 == null && l2 == null) return null;
        ListNode dummy = new ListNode(-1);
        ListNode pre = dummy;
        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                pre.next = l1;
                l1 = l1.next;
            }else{
                pre.next = l2;
                l2 = l2.next;
            }
            //更新pre
            pre = pre.next;
        }
        pre.next = (l1 == null) ? l2 : l1;
        return dummy.next;
    }
}