删除链表中倒数第n个节点

给定一个链表，删除链表中倒数第n个节点，返回链表的头节点。

样例

给出链表1->2->3->4->5->null和 n = 2.

删除倒数第二个节点之后，这个链表将变成1->2->3->5->null.

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @param n: An integer.
     * @return: The head of linked list.
     */
    ListNode *removeNthFromEnd(ListNode *head, int n) {
        // write your code here
        if(n==) return head;
        int res=;
        ListNode *p=head;
        while(p){
            res++;
            p=p->next;
        }
        ListNode *k=new ListNode();
        k->next=head;
        head=k;
        ListNode *q=head->next;
        for(int i=;i<=res;i++){
            if(res-i+==n){
                k->next=q->next;
                break;
            }
            k=q;
            q=q->next;
        }
        head=head->next;
        return head;
    }
};