【LeetCode】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

分治解决法（java版本）：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        int length = lists.length;
        if (length == 0) {
            return null;
        }
        if (length == 1) {
            return lists[0];
        }
        if (length == 2) {
            return mergeTwoLists(lists[0],lists[1]);
        }
        int mid = lists.length/2;
        ListNode[] listsLeft = new ListNode[mid];
        for (int i = 0;i < mid;i++) {
            listsLeft[i] = lists[i];
        }
        ListNode[] listsRight = new ListNode[length-mid];
        for (int i = 0;i < length-mid;i++) {
            listsRight[i] = lists[mid+i];
        }
        return mergeTwoLists(mergeKLists(listsLeft),mergeKLists(listsRight));
    }
    public ListNode mergeTwoLists(ListNode l1,ListNode l2) {
        if (null == l1) {
            return l2;
        } 
        if (null == l2) {
            return l1;
        }
        ListNode head = null;
        if (l1.val <= l2.val) {
            head = l1;
            head.next = mergeTwoLists(l1.next,l2);
        } else {
            head = l2;
            head.next = mergeTwoLists(l1,l2.next);
        }
        return head;
    }
}

其核心的解决思路就是将原本较为复杂的多个List合并转换为2个List合并，然后对最基本的情况进行处理，向上递归就可以解决复杂的问题。