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) {
if (head == NULL)
return NULL;
ListNode *p = head, *q = head;
int index = 0;
while(index < n)
{
p = p->next;
index++;
}
if(p == NULL)
return head->next;
else
{
while(p->next)
{
p = p->next;
q = q->next;
}
}

q->next = q->next->next;
return head;
}
};