3从尾到头打印链表

/**
*  struct ListNode {
*        int val;
*        struct ListNode *next;
*        ListNode(int x) :
*              val(x), next(NULL) {
*        }
*  };
*/
class Solution {
public:
    vector<int> printListFromTailToHead(ListNode* head) {
        vector<int> result;
        if(head == nullptr){
            return result;
        }
        stack<int> s;
        ListNode* pHead = head;
        while(pHead != nullptr){
            s.push(pHead -> val);
            pHead = pHead -> next;
        }
        int sz = s.size();
        for(int i = 0;i < sz;++i){
            result.push_back(s.top());
            s.pop();
        }
        return result;
    }
};

class Solution {
public:
    void helper(ListNode* head,vector<int> &result){
        if(head == nullptr){
            return;
        }
         
        helper(head -> next,result);
        result.push_back(head -> val);//一定要把位置放在这里
    }
    vector<int> printListFromTailToHead(ListNode* head) {
        vector<int> result;
        if(head == nullptr){
            return result;
        }
        helper(head,result);
        return result;
    }
};