Remove Duplicates from Sorted List II

Remove Duplicates from Sorted List II

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.

/**
 * 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
        ListNode *fast = head, *slow = NULL, *last = NULL, *newhead = NULL;
        int count = 1;
        
        while (fast) {
            if (last && fast->val == last->val) {
                ++count;
            }
            else if (last && fast->val != last->val) {
                if (count == 1) {
                    if (!slow) {
                        slow = last;
                        newhead = slow;
                    }
                    else {
                        slow->next = last;
                        slow = slow->next;
                    }
                }
                else {
                    count = 1;
                }
            }
            
            last = fast;
            fast = fast->next;
        }
        
        //check if the final node in the original list needs to be added
        if (last && slow && last->val != slow->val && count == 1) {
            slow->next = last;
            slow = slow->next;
        }
        else if (!slow && last && count == 1) {
            //only one node
            return last;
        }
        
        //close the new list
        if (slow) {
            slow->next = NULL;
        }
        
        return newhead;
    }
};