class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        // sentinel
        ListNode sentinel = new ListNode(0, head);

        // predecessor = the last node 
        // before the sublist of duplicates
        ListNode pred = sentinel;
        
        while (head != null) {
            // if it's a beginning of duplicates sublist 
            // skip all duplicates
            if (head.next != null && head.val == head.next.val) {
                // move till the end of duplicates sublist
                while (head.next != null && head.val == head.next.val) {
                    head = head.next;    
                }
                // skip all duplicates
                pred.next = head.next;     
            // otherwise, move predecessor
            } else {
                pred = pred.next;    
            }
                
            // move forward
            head = head.next;    
        }  
        return sentinel.next;
    }
}