Remove Nth Node From End of List

class Solution {
public:
ListNode* removeNthFromEnd(ListNode* head, int n) {
if(n<=0||head==NULL) return head;
ListNode prehead(0);
prehead.next=head;
ListNode *p=head;
int length=0,i;
while(p->next)
{
length++;
p=p->next;
}
p=&prehead;
length++;
if(n>length)n=n%length;
i=length-n;

while(i--)p=p->next;
if(n==1) p->next=NULL;
else  p->next=p->next->next;
return prehead.next;
}
};

class Solution {
public:
ListNode* removeNthFromEnd(ListNode* head, int n) {
if(n<=0||head==NULL) return head;
ListNode prehead(0);
prehead.next=head;
ListNode *p=&prehead;
head=&prehead;
while(n--)
{
p=p->next;
}
while(p->next)
{
p=p->next;
head=head->next;
}
head->next=head->next->next;

return prehead.next;
}
};