# Merge k Sorted Lists

## Problem

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 <= 104
- 0 <= lists[i].length <= 500
- -104 <= lists[i][j] <= 104
- lists[i] is sorted in ascending order.
- The sum of lists[i].length will not exceed 104.

## Solution

This is a heap problem.

```
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode mergeKLists(ListNode[] lists) {
var q = new PriorityQueue<ListNode>((l, r) -> {
return Integer.compare(l.val, r.val);
});
for (var l : lists) {
if (l != null) {
q.add(l);
}
}
ListNode head = null;
ListNode curr = null;
while (!q.isEmpty()) {
var t = q.poll();
if (t.next != null) {
q.add(t.next);
}
if (head == null) {
head = t;
curr = head;
curr.next = null;
continue;
}
curr.next = t;
curr = t;
}
return head;
}
}
```