class Solution {
public:
    vector<int> ret;
    stack<TreeNode*> s;
    TreeNode* temp=nullptr;
    vector<int> preorderTraversal(TreeNode* root) {
        // write code here
        if(root == nullptr) return ret;
        s.push(root);
        while(!s.empty())
        {
            temp = s.top();
            ret.push_back(temp->val);
            s.pop();
            if(temp->right)
                s.push(temp->right);
            if(temp->left)
                s.push(temp->left); 
        }
        return ret;
    }
};