/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 * };
 */

#include <cstddef>
#include <vector>
class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
    vector<int> preorderTraversal(TreeNode* root) {
        // write code here
        vector<int> res;
        preorder(root, res);
        return res;
    }
    void preorder (TreeNode* root, vector<int>& res) {
        if (!root) {
            return;
        }
        res.push_back(root->val);
        preorder(root->left, res);
        preorder(root->right, res);
    }
};