LeetCode - Remove Duplicates from Sorted List II

RemoveDuplicatesfromSortedListII.java

//
public ListNode deleteDuplicates(ListNode head) {
        if(head == null) return head;
        ListNode dummyNode = new ListNode(0);
        dummyNode.next = head;
        ListNode pre = dummyNode;
        while(head != null && head.next != null){
            if(head.val == head.next.val){
                while(head.next != null && head.val == head.next.val){
                    head = head.next;
                }
                pre.next = head.next;
            } else {
                pre = pre.next;
            }
            head = pre.next;
        }
        return dummyNode.next;
    }
//
public ListNode deleteDuplicates(ListNode head) {
        if(head==null || head.next==null) return head;
        ListNode dummy = new ListNode(0);
        ListNode pre = dummy;
        pre.next = head;
        while(head != null && head.next != null){
            if(head.val!=head.next.val){
                pre = pre.next;
                head = head.next;
            }else{
                int val = head.val;
                while(head!=null && head.val==val){
                    head = head.next;
                }
                pre.next = head;
            }
        }
        return dummy.next;
    }
//
public ListNode deleteDuplicates(ListNode head) {
        if(head == null)
            return head;
        ListNode dummy = new ListNode(0);
        ListNode pre = dummy;
        pre.next = head;
        ListNode cur = head;
        while(cur != null){
            while(cur.next != null && cur.val == cur.next.val){
                cur = cur.next;
            }
            if(pre.next == cur)
                pre = pre.next;
            else
                pre.next = cur.next;
            cur = cur.next;
        }
        return dummy.next;
    }