144. Binary Tree Preorder Traversal

class Solution {
public:
    vector<int>res;
    void preorder(TreeNode* root){
        if(!root)return;
        res.push_back(root->val);
        preorder(root->left);
        preorder(root->right);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        preorder(root);
        return res;
    }
};