LeetCode 23. Merge k Sorted Lists

Question

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.

https://leetcode.com/problems/merge-k-sorted-lists/

• Solution1

class Solution {
    //pq,time O(nlogk), space O(K)
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists.length == 0) return null;
        ListNode dummy = new ListNode(0);
        PriorityQueue<ListNode> pq = new PriorityQueue<>((a, b) -> a.val - b.val);
        for (int i = 0; i < lists.length; i++)
            if (lists[i] != null) pq.offer(lists[i]);//only first element will be insert to the pq
        ListNode cur = dummy;
        while(!pq.isEmpty()){
            cur.next = pq.poll();
            cur = cur.next;
            if (pq.isEmpty()) break; //last cycle dont need work
            if (cur.next != null) pq.offer(cur.next);
        }
        return dummy.next;
    }
}

Complexity:

Time complexity: O(nklogk)

Space complexity: O(k)