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.

Solution:

```
class Solution {
public ListNode mergeKLists(ListNode[] lists) {
PriorityQueue<Integer> minheap = new PriorityQueue<>();
for(ListNode head:lists)
{
while(head!=null)
{
minheap.add(head.val);
head=head.next;
}
}
ListNode dummy = new ListNode(-1);
ListNode head = dummy;
while(!minheap.isEmpty())
{
head.next=new ListNode(minheap.remove());
head=head.next;
}
return dummy.next;
}
}
```

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