"""
23. Merge k Sorted Lists
Hard

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

 

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

 

Constraints:

    k == lists.length
    0 <= k <= 10^4
    0 <= lists[i].length <= 500
    -10^4 <= lists[i][j] <= 10^4
    lists[i] is sorted in ascending order.
    The sum of lists[i].length won't exceed 10^4.

http://leetcode.com/problems/merge-k-sorted-lists/
"""
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeKLists(self, lists: List[ListNode]) -> ListNode:
        if not lists:
            return None
        nodes = []
        for n in lists:
            while n:
                nodes.append(n)
                n = n.next
        nodes.sort(key=lambda x: x.val)
        root = None
        head = None
        for n in nodes:
            if not root:
                root = n
                head = root
            else:
                head.next = n
                head = head.next
        return root
