LeetCode Problem : Binary Tree Preorder Traversal

Problem

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

vector<int> preorderTraversal(TreeNode *root) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    vector<int> result;
    stack<TreeNode *> st;
    if(root)
        st.push(root);
    while(!st.empty()){
        TreeNode *node = st.top();
        st.pop();
        result.push_back(node->val);
        if(node->right)
            st.push(node->right);
        if(node->left)
            st.push(node->left);
    }
    return result;
}