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;
}
};

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