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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
#include<stack>
using namespace std;
class BSTIterator {
    stack<TreeNode*> st;
    
public:
    BSTIterator(TreeNode *root) {
        if(root)
            putLeftChild(root);
        
    }

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

    /** @return the next smallest number */
    int next() {
        TreeNode* top = st.top();
        st.pop();
        if(top->right)
            putLeftChild(top->right);
        
        return top->val;        
    }
    
    void putLeftChild(TreeNode *root){
        while(root){
            st.push(root);
            root = root->left;
        }        
    }
};
/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */