Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* root = head;
        if (head  == NULL || head->next == NULL)
        return root;
        while (head->next != NULL)
        {
            if(head ->val ==head->next->val)
            {head->next = head->next->next;
            }
            else head = head->next;
           
        }
       return  root;
    }
};