144. Binary Tree Preorder Traversal (Tree, Stack)

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?

class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> result;
        if(!root) return result;

        stack<TreeNode* > treeStack;
        treeStack.push(root);
        TreeNode* current;
        while(!treeStack.empty())
        {
            current = treeStack.top();
            treeStack.pop();
            result.push_back(current->val);
            if(current->right) treeStack.push(current->right);
            if(current->left) treeStack.push(current->left);
        }
        return result;
    }
};