23 Merge k Sorted Lists - Hard

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

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6

Code

MinHeap

/**
 * 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 {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists.length == 0 || lists == null)
            return null;
        
        Queue<ListNode> q = new PriorityQueue<ListNode>(lists.length,
        new Comparator<ListNode>(){
            public int compare(ListNode l1, ListNode l2){
                return l1.val - l2.val;
            }
        });
        
        for(int i = 0; i < lists.length; i++){
            if(lists[i] != null)
                q.offer(lists[i]);
        }
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        
        while(!q.isEmpty()){
            ListNode node = q.poll();
            cur.next = node;
            cur = node;
            if(node.next != null){
                q.offer(node.next);
            }
        }
        
        return dummy.next;
        
    }
}