Binary Search Tree Iterator

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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
private:
    stack<TreeNode *> st;
   
public:
    BSTIterator(TreeNode *root) {
        while (root != NULL) {
            st.push(root);
            root = root -> left;
        }
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return !st.empty();
    }

    /** @return the next smallest number */
    int next() {
        int val;
        if (!st.empty())  {
            TreeNode * trav = st.top();
            val = trav -> val;
            st.pop();
            if (trav -> right) {
                TreeNode *tmp = trav -> right;
                while (tmp != NULL) {
                    st.push(tmp);
                    tmp = tmp -> left;
                }
            }
        }
        return val;
    }
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */