Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

 /**
  * 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;
         help(root,res);
         return res;
     }
     void help(TreeNode *root,vector<int> &res)
     {
         if(root!=NULL)
         {
             res.push_back(root->val);
             help(root->left,res);
             help(root->right,res);

         }
     }
 };
 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode *root)
     {
         vector<int> res;
         stack<TreeNode* > s;
         s.push(root);
         while(!s.empty())
         {
             TreeNode *now=s.top();
             s.pop();
             if(now!=NULL)
             {
                 res.push_back(now->val);
                 s.push(now->right);
                 s.push(now->left);
             }
         }
         return res;
     }

 };