LeetCode Binary Search Tree Iterator

class BSTIterator {
private:
    TreeNode* current;
    stack<TreeNode*> nodeStack;
public:
    BSTIterator(TreeNode *root) {
        current = root;
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        if (nodeStack.empty() && current == NULL) {
            return false;
        } else {
            return true;
        }
    }

    /** @return the next smallest number */
    int next() {
        while (current != NULL) {
            nodeStack.push(current);
            current = current->left;
        }
        // node current must be null,
        // the last one pushed in the stack must be the smallest one
        
        TreeNode* last = nodeStack.top();
        nodeStack.pop();
        
        // next we will start from its right sub-tree to find the smallest one
        current = last->right;

        return last->val;
    }
};