LeetCode: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?

/**
 * 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<int> preorderTraversal(TreeNode *root) {
        stack<TreeNode*> stack;
        vector<int> res;
        if (!root){return res;}
        stack.push(root);
        TreeNode* head=root;
        while (!stack.empty()){
            TreeNode* top = stack.top(); 
            res.push_back(top->val);  
            stack.pop();
            if (top->right!=NULL){stack.push(top->right);} // 先右边入stack
            if (top->left!=NULL){stack.push(top->left);}
        }
        return res;
    }
};