leetcode.19----------Remove Nth Node From End of List

nth

For example,

1->2->3->4->5, and n.

   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.

Runtime: 9 ms

class Solution{
public:
  ListNode *removeNthFromEnd(ListNode *head, int n)
  {
    if (head == NULL)
      return NULL;

    ListNode*p1 = head;
    ListNode*p2 = head;
    ListNode*pre = NULL;
    while (n-1 != 0)
    {
      p1 = p1->next;
      n--;
    }

    while (p1->next)
    {
      pre = p2;
      p2 = p2->next;
      p1 = p1->next;
    }

    if (p2 == head)
      head = head->next;
    else
    {
      pre->next = p2->next;
      delete p2;
    }
    return head;
  }
};