/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class BSTIterator {

public:

    BSTIterator(TreeNode *root) {

        while(root) {

            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() {

        TreeNode* top = st.top();

        int val = top->val;

        st.pop();

        top = top->right;

        while(top) {

            st.push(top);

            top = top->left;

        }

        return val;

    }

private:

    stack<TreeNode*> st;

};

/**

 * Your BSTIterator will be called like this:

 * BSTIterator i = BSTIterator(root);

 * while (i.hasNext()) cout << i.next();

 */