Partition List

public ListNode partition(ListNode head, int x) {
// Start typing your Java solution below
// DO NOT write main() function
if(head == null) return null;
ListNode tmp = new ListNode(-1);
tmp.next = head;
ListNode current = head;
ListNode previous = tmp;
ListNode next = null;
while(current.next != null && current.val < x) {
previous = current;
current = current.next;
}
while(current.next != null) {
next = current.next;
if(next.val < x) {
current.next = next.next;
next.next = previous.next;
previous.next = next;
previous = next;
}else {
current = current.next;
}
}
head = tmp.next;
tmp.next = null;
return head;
}