力扣——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.length == 0)
            return null;
        int k = lists.length;
        while (k > 1) {            
            for (int i = 0; i < k / 2; i++) 
                lists[i] = mergeTwoLists(lists[i], lists[i + (k + 1) / 2]);
            k = (k + 1) / 2;
        }
        return lists[0];
    }
    
    private ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode root = new ListNode(0);
        ListNode node = root;
        while (list1 != null && list2 != null) {
            if (list1.val <= list2.val) {
                node.next = list1;
                list1 = list1.next;                
            } else {
                node.next = list2;
                list2 = list2.next;                  
            }
            node = node.next;
        }
        if (list1 == null)
            node.next = list2;
        if (list2 == null)
            node.next = list1;
        return root.next;        
    }
}