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.**Code (Java):**

/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class BSTIterator { private TreeNode p; private Stack<TreeNode> stack = new Stack<TreeNode>(); public BSTIterator(TreeNode root) { this.p = root; } /** @return whether we have a next smallest number */ public boolean hasNext() { return !stack.isEmpty() || p != null; } /** @return the next smallest number */ public int next() { while (p != null) { stack.push(p); p = p.left; } TreeNode curr = stack.pop(); p = curr.right; return curr.val; } } /** * Your BSTIterator will be called like this: * BSTIterator i = new BSTIterator(root); * while (i.hasNext()) v[f()] = i.next(); */

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