Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

// non-recursion version:

/**

 * 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> solution;
    // recursive is easy
    // let's do it without recursive
    // probably need to use stack
    vector<int> preorderTraversal(TreeNode *root) {
        if (root==NULL)
            return solution;
        stack<TreeNode *> mystack;
        TreeNode * node;
        mystack.push(root);
        while (!mystack.empty()){
            node = mystack.top();
            mystack.pop();
            
            if (node){
                solution.push_back(node->val);
                mystack.push(node->right);
                mystack.push(node->left);
            }
        }
        return solution;
    }
};