LeetCode_Remove Nth Node From End of List

Given a linked list, remove the nth 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.

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *removeNthFromEnd(ListNode *head, int n) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode *p, *q, *pre;
p = q = head;
int i = 1;
while(q && i<n)
{
q = q->next;
i++;
}
if(i < n) return head;

while(q->next)
{
pre = p;
p = p->next;
q = q->next;
}

if(p == head)
head = head->next;
else
pre->next = p->next;

delete p;
return head;

}
};