23. Merge k Sorted Lists

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

/** * 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> heap = new PriorityQueue<ListNode>(lists.length, ListNodeComparator);for(int i=0; i<lists.length; i++){if(lists[i] != null){heap.add(lists[i]);}}ListNode dummy = new ListNode(0);ListNode tail = dummy;while(!heap.isEmpty()){ListNode head = heap.poll();tail.next = head;tail = head;if(head.next != null){heap.add(head.next);}}return dummy.next;}private Comparator<ListNode> ListNodeComparator = new Comparator<ListNode>(){public int compare(ListNode n1, ListNode n2){if(n1 == null){return 1;}if(n2 == null){return -1;}return n1.val - n2.val;}};}