如何在C++中将preOrdered树键数放入数组中

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

void preOrder(TreeNode* root, int arr[], int& index) {
    if (root == nullptr) {
        return;
    }
    arr[index++] = root->val;
    preOrder(root->left, arr, index);
    preOrder(root->right, arr, index);
}

int* preOrderTraversal(TreeNode* root, int& size) {
    int* arr = new int[size];
    int index = 0;
    preOrder(root, arr, index);
    size = index;
    return arr;
}

int main() {
    // 创建一个示例树
    TreeNode* root = new TreeNode(1);
    root->left = new TreeNode(2);
    root->right = new TreeNode(3);
    root->left->left = new TreeNode(4);
    root->left->right = new TreeNode(5);

    int size = 5;
    int* arr = preOrderTraversal(root, size);

    // 输出结果数组
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    // 释放内存
    delete[] arr;
    delete root->left->left;
    delete root->left->right;
    delete root->left;
    delete root->right;
    delete root;

    return 0;
}