Remove Duplicates from Sorted List

/*

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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        map<int,int> hashMap;
        if(head==NULL) return head;
       
        ListNode *prep = head;
        ListNode *curr = head->next;
        hashMap[prep->val]=1;
        while(curr!=NULL){
          if(hashMap.find(curr->val)==hashMap.end()){
            hashMap[curr->val]=1;
            prep = curr;
            curr = curr->next;
            } else {
              curr = curr->next;
              prep->next = curr;
            }    
        }
        return head;
    }
};