Remove Nth Node From End of List

ListNode *removeNthFromEnd(ListNode *head, int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(n <= 0 || head == NULL)
return head;
int len = 0;
ListNode *tmp = head;
while(tmp != NULL){
len++;
tmp = tmp->next;
}
if(n > len)
return head;
if(n == len)
return head->next;
len = len - n;
tmp = head;
while(len > 1){
len--;
tmp = tmp->next;
}
tmp->next = tmp->next->next;
return head;
}