leetcode #144 in cpp

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?

Code:

/**
 * 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*> stk; 
        if(root) stk.push(root);
        TreeNode*cur; 
        while(!stk.empty()){
            cur = stk.top();
            stk.pop();
            res.push_back(cur->val);
            if(cur->right) stk.push(cur->right);
            if(cur->left) stk.push(cur->left);
        }
        return res; 
        
    }
};