//0033反转链表
class Solution {
public:
    vector<int>buff;
    void dfs(ListNode * root)
    {
        if (root == NULL)
            return ;
        else {
            buff.push_back(root -> val);
            dfs(root -> next);
        }
    }
    ListNode* reverseList(ListNode* head) {
        buff.clear();
        ListNode *ans = head;
        dfs(head);
        int i = buff.size();
        while(head) {
            head -> val = buff[--i];
            head = head-> next;
        }
        return ans;
    }
};