text: // 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:
    void help(TreeNode *root) {
      if (!root) { return; }
  
      help(root->left);
      v_.push_back(root->val);
      help(root->right);
    }
  
  private:
    std::vector<int> v_;
    int i_;
  
  public:
      BSTIterator(TreeNode *root) {
        i_ = 0;
        help(root);
      }
  
      /** @return whether we have a next smallest number */
      bool hasNext() {
        return i_ < v_.size();
      }
  
      /** @return the next smallest number */
      int next() {
        return v_[i_++];
      }
  };
  
  /**
   * Your BSTIterator will be called like this:
   * BSTIterator i = BSTIterator(root);
   * while (i.hasNext()) cout << i.next();
   */
  