/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
#include <vector>
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
    
    vector<int> ans;
    void dfs(TreeNode* root)
    {
        if(!root)
            return;
        
        ans.emplace_back(root->val);
        dfs(root->left);
        dfs(root->right);
    }

    vector<int> preorderTraversal(TreeNode* root) {
        // write code here
        dfs(root);
        return ans;
    }
};