Flatten BST to a sorted list

void inOrder(TreeNode<int>* root, vector<int> &v){
    if(root == NULL){
        return ;
    }
    inOrder(root->left,v);
    v.push_back(root->data);
    inOrder(root->right,v);
}

TreeNode<int>* flatten(TreeNode<int>* root)
{
    vector<int> v;
    inOrder(root,v);
    int n = v.size();
    TreeNode<int>* newRoot = new TreeNode<int>(v[0]);
    TreeNode<int>* curr = newRoot;
    for(int i=1;i<n;i++){
        TreeNode<int>* temp = new TreeNode<int>(v[i]);
        curr->left = NULL;
        curr->right = temp;
        curr = temp;
    }
    curr->left = NULL;
    curr->right = NULL;
    return newRoot;
}