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.

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {

        if(head==NULL){
            return NULL;
        }

        ListNode* dummyHead=new ListNode(0);
        dummyHead->next=head;
        //p->target...->q(NULL)
        ListNode* p=dummyHead;

        ListNode* q=dummyHead;
        for(int i=0;i<n+1;i++){
            //ensure q !=NULL
            assert(q);
            q=q->next;
        } 

        //find node
        while(q != NULL){
            p=p->next;
            q=q->next;
        }

        //delete node
        ListNode* delNode=p->next;
        p->next=delNode->next;
        delete delNode;

        ListNode* ret=dummyHead->next;
        delete dummyHead;
        return ret;
    }
};