/**
 * https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list/
 *
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        
        // 1. 
        if(head == null || head.next == null){
            return head;
        }
        
        // 2.  left  right 
        ListNode left = head;
        ListNode right = head.next;
        
        // 3.  right 
        //  -  right left 
        //  -  left.next  right left  right 
        while(right!=null){
            if(left.val == right.val){
                right=right.next;
            }else{
                left.next = right;
                left = right;
                right = right.next;
            }
        }
        
        // 4.  right null
        //  - left.next  null
        //  - left   null 
        //  so =>
        left.next = null;
        return head;
    }
}