Leetcode-Remove Nth Node From End of List

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.

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;    }};