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;    }