Topics Construct a strict binary tree

TreeNode<int> *constructTreeHelper(vector<int> &pre, vector<char> &typeNL, int &index_ptr, int &n)
{
    // Store the current value of index in pre[].
    int index = index_ptr;

    if (index == n)
    {
        return NULL;
    }

    /*
        Allocate memory for this node and 
        increment index for subsequent recursive calls.
    */
    TreeNode<int> *temp = new TreeNode<int>(pre[index]);

    (index_ptr)++;

    /*
        If this is an internal node, construct left 
        and right subtrees and link the subtrees.
    */
    if (typeNL[index] == 'N')
    {
        temp->left = constructTreeHelper(pre, typeNL, index_ptr, n);
        temp->right = constructTreeHelper(pre, typeNL, index_ptr, n);
    }

    return temp;
}

TreeNode<int> *constructTree(vector<int> &pre, vector<char> &typeNL, int &n)
{
    /*
        Index pointer is used to update index values in recursive calls,
        index must be initially passed as 0.
    */
    int index = 0;

    return constructTreeHelper(pre, typeNL, index, n);
}

TreeNode<int> *constructSBT(vector<int> &pre, vector<char> &typeNL)
{
    int n = pre.size();

    TreeNode<int> *root = constructTree(pre, typeNL, n);

    return root;
}