Leetcode-Binary Tree Inorder Traversal

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

For example:
Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [1,3,2].

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

/**
 * 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 inorderTraversal(TreeNode* root) {
        vector ans;
        if(!root)
            return ans;
        stack res;
        res.push(root);
        while(!res.empty()){
            TreeNode* Node = res.top();
            if(Node->left){
                res.push(Node->left);
                Node->left = NULL;
            }
            else{
                ans.push_back(Node->val);
                res.pop();
                if(Node->right)
                    res.push(Node->right);
            }
        }
        return ans;
    }
};