/**
knakul853
 */
class Solution {
public:
    vector<int>ans;
    vector<int> preorderTraversal(TreeNode* root) {

        stack<TreeNode*> st;
        vector<int>ans;

        st.push(root);
        while(!st.empty())
        {
            TreeNode* node = st.top();st.pop();
            if( !node )continue;
            ans.push_back(node->val);

             if( node->right != NULL ) st.push(node->right);
             if ( node-> left != NULL ) st.push(node->left);


        }

        return ans;

    }
};
class Solution {
public:
    vector<int>ans;
    vector<int> preorderTraversal(TreeNode* root) {

        if(!root) return {};

        ans.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);

        return ans;

    }
};