Binary Search Tree Iterator leetcode

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

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class BSTIterator {
private:
    TreeNode *current = NULL;
    stack<TreeNode*> s;
public:
    BSTIterator(TreeNode *root) {
         // initialize the current pointer
        current = root;
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        while(current){
            s.push(current);
            current = current->left;
        }
        return !s.empty();
    }

    /** @return the next smallest number */
    int next() {
        TreeNode* node = s.top();
        s.pop();
        current = node->right;
        return node->val;
    }
};