Merge k Sorted Lists

Merge k Sorted Lists

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

Note: Merge sort was used.
Time complexity: O(nlgk).

/** 
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ArrayList<listnode> lists) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (lists.size() == 0) return null;
        ArrayList<listnode> klist = new ArrayList<listnode>();
        for (int i = 0; i &lt; lists.size(); i++) {
            ListNode node = lists.get(i);
            if (node != null) {
                klist.add(node);
            }
        }
        Collections.sort(klist, new Comparator<listnode>() {
            public int compare(ListNode n1, ListNode n2) {
                return n1.val - n2.val;
            }
        });
        ListNode root = null;
        ListNode node = null;
        
        while (!klist.isEmpty()) {
            
            if (root == null) {
                root = klist.get(0);
                node = root;
            }
            else {
                node.next = klist.get(0);
                node = node.next;
            }
            
            klist.set(0, klist.get(0).next);
            if (klist.get(0) == null) {
                klist.remove(0);
            }
            
            if (klist.isEmpty()) {
                break;
            }
            
            Collections.sort(klist, new Comparator<listnode>() {
                public int compare(ListNode n1, ListNode n2) {
                    return n1.val - n2.val;
                }
            });
            
            
        }
        return root;
    }
}