Given a linked list, remove the n^th node from the end of list and return its head.

For example,

   Given linked list: 1->2->3->4->5, and n = 2.

   After removing the second node from the end, the linked list becomes 1->2->3->5.

Note:
Given n will always be valid.
Try to do this in one pass.

ListNode *removeNthFromEnd(ListNode *head, int n) {
    ListNode *p1,*p2;
    int i=0;
    if(head->next==NULL) return NULL;
    for(p1=head,p2=head;p1->next!=NULL;i++,p1=p1->next){
        if(i>=n) p2=p2->next;
    }
    if(i<n) head=p2->next;
    else
        p2->next=p2->next->next;
    return head;
}

Remove Nth Node From End of List