MergeKSortedLists.java

// 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 MergeKSortedLists {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists==null||lists.length==0) {
            return null;
        }
        
        PriorityQueue<ListNode> queue= new PriorityQueue<ListNode>(lists.length,new Comparator<ListNode>(){
            @Override
            public int compare(ListNode o1,ListNode o2){
                if (o1.val<o2.val) {
                    return -1;
                } else if (o1.val==o2.val) {
                    return 0;
                } else {
                    return 1;
                }
            }
        });
        
        ListNode dummy = new ListNode(0);
        ListNode tail=dummy;
        
        for (ListNode node:lists) {
            if (node!=null) {
                queue.add(node);
            }
        }

        while (!queue.isEmpty()){
            tail.next=queue.poll();
            tail=tail.next;

            if (tail.next!=null) {
                queue.add(tail.next);
            }
        }

        return dummy.next;
    }
}