Leetcode#19 Remove nth node from end of list

class Solution {
    public:
        ListNode* removeNthFromEnd(ListNode* head, int n) {
            ListNode* fast = head;
            ListNode* slow = head;
            ListNode* slow_pre = NULL;

            while (n > 1) {
                fast = fast->next;
                n--;
            }

            while(fast->next != NULL) {
                fast = fast->next;
                slow_pre = slow;
                slow = slow->next;
            }

            if (slow_pre == NULL) {
                ListNode* slow_next = slow->next;
                slow->next = NULL;
                return slow_next;
            } else {
                slow_pre->next = slow->next;
                return head;
            }
        }
};