Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

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Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0) return null;


        Queue<ListNode> queue = new PriorityQueue<ListNode>((a, b) -> (Integer.compare(a.val, b.val)));
        ListNode dummy = new ListNode(0);
        ListNode iter = dummy;
        for(ListNode node: lists) {
            if(node != null) queue.offer(node);
        }

        while(!queue.isEmpty()) {
            ListNode node = queue.poll();
            if(node.next != null) queue.offer(node.next);
            iter.next = node;
            iter = iter.next;
        }
        iter.next = null;
        return dummy.next;
    }
}