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].

Note: Recursive solution is trivial, could you do it iteratively?

Solution : 


CODE:
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector preorderTraversal(TreeNode *root) {
      
       vector result;
       stack s;
       if(root!=NULL)
       s.push(root);
       while(!s.empty()){
           TreeNode *curr = s.top();
           result.push_back(curr->val);
           s.pop();
           if(curr->right)
           s.push(curr->right);
           if(curr->left)
           s.push(curr->left);
       }
       return result;
    }
};