```
public ListNode mergeKLists(ListNode[] lists) {
if (lists == null || lists.length == 0) {
return null;
}
int size = lists.length;
while (size > 1) {
int left = 0, right = size - 1;
while (left < right) {
lists[left] = mergeTwoLists(lists[left], lists[right]);
left++;
right--;
}
size = (size + 1) / 2;
}
return lists[0];
}
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null) return l2;
if (l2 == null) return l1;
ListNode cur = new ListNode(0);
ListNode newhead = cur;
while (l1 != null && l2 != null) {
if (l1.val <= l2.val) {
cur.next = l1;
l1 = l1.next;
} else {
cur.next = l2;
l2 = l2.next;
}
cur = cur.next;
}
if (l1 == null) {
cur.next = l2;
} else {
cur.next = l1;
}
return newhead.next;
}
```