priorityQueue --minHeap O(n * k log(k)) time

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if(lists == null || lists.length == 0) {
            return null;
        }

        PriorityQueue<ListNode> minHeap = new PriorityQueue<ListNode>(lists.length, new Comparator<ListNode>() {
            @Override
            public int compare(ListNode node1, ListNode node2) {
                if (node1.val == node2.val) {
                    return 0;
                } else {
                    return node1.val > node2.val ? 1 : -1;
                }
            }
        });

        for(int i = 0; i < lists.length; i++) {
            if(lists[i] != null) {
                minHeap.offer(lists[i]);
            }
        }
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;

        while(!minHeap.isEmpty()) {
            ListNode node = minHeap.poll();
            if(node.next != null) {
                minHeap.offer(node.next);
            }

            cur.next = node;
            cur = cur.next;
        }

        return dummy.next;
    }

}

Binary merge sort O ( n * k * log k) time,

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) {
            return null;
        }

        return mergeSort(lists, 0, lists.length - 1);

    }

    private ListNode mergeSort(ListNode[] lists, int left, int right) {
        if (left == right) {
            return lists[left];
        }

        int mid = left + (right - left) / 2;
        ListNode lnode = mergeSort(lists, left, mid);
        ListNode rnode = mergeSort(lists, mid + 1, right);
        return mergeTwoList(lnode, rnode);
    }

    private ListNode mergeTwoList(ListNode lnode, ListNode rnode) {
        if(lnode == null && rnode == null) {
            return null;
        } else if (lnode != null && rnode == null) {
            return lnode;
        } else if (rnode != null && lnode == null) {
            return rnode;
        }

        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;

        while(lnode != null && rnode != null) {
            if(lnode.val <= rnode.val) {
                cur.next = lnode;
                cur = cur.next;
                lnode = lnode.next;
            } else {
                cur.next = rnode;
                cur = cur.next;
                rnode = rnode.next;
            }
        }
        if (lnode != null) {
            cur.next = lnode;
        } 
        if (rnode != null) {
            cur.next = rnode;
        }
        return dummy.next;
    }
}