Leetcode: Remove Nth Node From End of List in C++

Given a linked list, remove the n^th node from the end of list and return its head.
Solution:
ListNode *removeNthFromEnd(ListNode *head, int n) {
        if(!head)
        return NULL;
        ListNode* fast = head;
        for(int i=0;i<n;i++)
           fast = fast->next;
        ListNode* slow = head;
        ListNode* pre = slow;
        if(!fast)
        {
            head = head->next;
            delete slow;
            return head;
        }
        while(fast)
        {
            fast = fast->next;
            pre = slow;
            slow = slow->next;
        }
            pre->next = slow->next;
            delete slow;
            return head;
    }