public ListNode RemoveNthFromEnd(ListNode head, int n)
{
if (head == null || n < 0 ) {
return head;
}
ListNode pre = head, aft = null;
for (int i = 0; i < n; ++i) {
pre = pre.next;
}
while (pre != null) {
pre = pre.next;
if (aft == null) {
aft = head;
} else {
aft = aft.next;
}
}
if (aft == null) {
head = head.next;
} else {
aft.next = aft.next.next;
}
return head;
}
public ListNode(int x)
{
val = x;
next = null;
}