<p>You are given an array of <code>k</code> linked-lists <code>lists</code>, each linked-list is sorted in ascending order.</p>

<p><em>Merge all the linked-lists into one sorted linked-list and return it.</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> lists = [[1,4,5],[1,3,4],[2,6]]
<strong>Output:</strong> [1,1,2,3,4,4,5,6]
<strong>Explanation:</strong> The linked-lists are:
[
  1-&gt;4-&gt;5,
  1-&gt;3-&gt;4,
  2-&gt;6
]
merging them into one sorted list:
1-&gt;1-&gt;2-&gt;3-&gt;4-&gt;4-&gt;5-&gt;6
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> lists = []
<strong>Output:</strong> []
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> lists = [[]]
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>k == lists.length</code></li>
	<li><code>0 &lt;= k &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= lists[i].length &lt;= 500</code></li>
	<li><code>-10<sup>4</sup> &lt;= lists[i][j] &lt;= 10<sup>4</sup></code></li>
	<li><code>lists[i]</code> is sorted in <strong>ascending order</strong>.</li>
	<li>The sum of <code>lists[i].length</code> will not exceed <code>10<sup>4</sup></code>.</li>
</ul>

from heapq import heapify, heappush, heappop
# Definition for singly-linked list.
class ListNode2:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
    def __lt__(self,other):
            return self.val < other.val

class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        solution = []
        lists2 = []
        for node in lists:
            if node:
                new_node = ListNode2(node.val)
                lists2.append(new_node)
                while node.next:
                    node = node.next
                    new_node.next = ListNode2(node.val)
                    new_node = new_node.next
        heapify(lists2)
        while len(lists2)>0:
            l = heappop(lists2)
            solution.append(l.val)
            if l.next:
                heappush(lists2,l.next)
        if len(solution) == 0:
            return None
        root_node = ListNode(solution[0])
        node = root_node
        for item in solution[1:]:
            node.next = ListNode(item)
            node = node.next
        return root_node
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