给定一个二叉树，返回它的 前序 遍历。

示例:

输入: [1,null,2,3]

   1

    \

     2

    /

   3 

输出: [1,2,3]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

 /**

  * Definition for a binary tree node.

  * struct TreeNode {

  *     int val;

  *     TreeNode *left;

  *     TreeNode *right;

  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

  * };

  */

 class Solution {

 public:

     vector<int> preorderTraversal(TreeNode* root) {

         vector<int> res;

         stack<TreeNode*> s;

         TreeNode* node = root;

         while(node || s.size()){

             if(node == NULL){

                 node = s.top();

                 s.pop();

             }

             res.push_back(node->val);

             if(node->right)

                 s.push(node->right);

             node = node->left;

         }

         return res;

     }

 };