MergeKSortedListsA.java

/**
 * 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 {

    // TC:  Klogk
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length ==0){
            return null;
        }

        Comparator<ListNode> customCompare = new Comparator<ListNode>(){
            @Override
            public int compare(ListNode a, ListNode b){
                return a.val -b.val;
            }
        };

        PriorityQueue<ListNode> pq = new PriorityQueue<ListNode> (customCompare);

        for(ListNode list : lists){
            if(list !=null){
                pq.offer(list);
            }
        }


        ListNode dummy = new ListNode(-1);
        ListNode t = dummy;

        while(!pq.isEmpty()){
            ListNode lowesetValNode = pq.poll();
            t.next = lowesetValNode;
            t   =   t.next;

            if(lowesetValNode.next!=null){
                pq.offer(lowesetValNode.next);
            }
        }

        return dummy.next;

    }
}