Leetcode -- Remove Duplicates from Sorted List II Delete duplicate characters in the SORT string (AC)

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given1->2->3->3->4->4->5, Return1->2->5.
Given1->1->1->2->3, Return2->3.

There is nothing to talk about. We can use recursive and iterative methods. We need to carefully consider various input situations. The code is as follows:

class Solution {public:    ListNode *deleteDuplicates(ListNode *head) {        if(head == NULL)            return NULL;        ListNode *first = head,*second = NULL,*result = NULL;        bool isDup = false;        while(first!=NULL)        {            isDup = false;            while(first->next != NULL && first->val == first->next->val)            {                isDup = true;                first = first->next;            }            if(!isDup)            {                if(second == NULL)                {                    second = first;                    if(result == NULL)                        result = second;                }                else                {                    second->next = first;                    second = second->next;                }            }            first = first->next;        }        if(second!=NULL)            second->next = NULL;        return result;    }};