Construct Binary Search Tree from Preorder Traversal

<-M 1008> Construct Binary Search Tree from Preorder Traversal

class Solution {
public:
    TreeNode* bstFromPreorder(vector<int>& preorder) {
        int i=0;
        return treeHelp(preorder, i, LONG_MAX);
    }

    TreeNode* treeHelp(vector<int> &preorder, int &i, long maxVal) {
        if(i == preorder.size() || preorder[i] > maxVal)
            return NULL;

        TreeNode *n = new TreeNode(preorder[i++]);

        n->left = treeHelp(preorder, i, n->val);
        n->right = treeHelp(preorder, i, maxVal);

        return n;
    }
};