[Leetcode 144, Medium] 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?

Analysis:

Solutions:

C++:

    vector<int> preorderTraversal(TreeNode *root) 
    {
        vector<int> rList;
        if(root == NULL)
            return rList;
            
        stack<TreeNode *> visited;
        TreeNode *pCurrNode = root;
        
        while(pCurrNode || !visited.empty()) {
            if(pCurrNode) {
                rList.push_back(pCurrNode->val);
                visited.push(pCurrNode);
                pCurrNode = pCurrNode->left;
            } else {
                pCurrNode = visited.top();
                visited.pop();
                pCurrNode = pCurrNode->right;
            }
        }
        return rList;
    }

Java:

Python: