Merge k Sorted Lists

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode mergeKLists(ListNode[] lists) {
if (lists == null) {
throw new IllegalArgumentException("aa");
}
if (lists.length == 0) {
return null;
}
return helper(lists, 0, lists.length - 1);
}
private ListNode helper(ListNode[] lists, int start, int end) {
if (start == end) {
return lists[start];
}
int mid = start + (end - start) / 2;
ListNode left = helper(lists, start, mid);
ListNode right = helper(lists, mid + 1, end);
ListNode node = merge(left, right);
return node;
}
private ListNode merge(ListNode left, ListNode right) {
ListNode dummy = new ListNode(0);
ListNode node = dummy;
while (left != null && right != null) {
if (left.val < right.val) {
node.next = left;
left = left.next;
} else {
node.next = right;
right = right.next;
}
node = node.next;
}
if (left != null) {
node.next = left;
}
if (right != null) {
node.next = right;
}
return dummy.next;
}
}