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;
    }
};