144.cpp

#include <bits/stdc++.h>
#include <vector>
using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution {
    vector<int> res;
private:
    void preorder(TreeNode* node) {
        if (!node) return;

        res.push_back(node->val);
        preorder(node->left);
        preorder(node->right);
    }

public:
    vector<int> preorderTraversal(TreeNode* root) {
        preorder(root);
        return res;
    }
};