LC.82. Remove Duplicates from Sorted List II

https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/description/
Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.
For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

time: o(n) complexity: o(1)

public ListNode deleteDuplicates(ListNode head) {
        if (head == null || head.next != null ) return head ;
        ListNode dummy = new ListNode(0) ;
        //注意，如果 CURR = HEAD 的话， 那第一个节点是没有办法删除掉的！ 所以要灵活的运用CURR
        ListNode curr = dummy;
        dummy.next = head ;
        /*
        *   1->2->3->3->4->4->5, return 1->2->5.
        * c c  c-----c
        * */
        while (curr.next != null && curr.next.next != null){
            if (curr.next.val == curr.next.next.val ){
                int val= curr.next.val;
                //凡事循环了，就要CHECK NPE
                while(curr.next != null && curr.next.val == val){
                    curr.next = curr.next.next ;
                }
            } else{
                curr = curr.next ;
            }
        }
        return dummy.next ;
    }