Binary Search Tree Iterator

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/

public class BSTIterator {

Stack<TreeNode> stack;
public BSTIterator(TreeNode root) {
stack = new Stack<>();
while (root != null) {
stack.push(root);
root = root.left;
}
}

/** @return whether we have a next smallest number */
public boolean hasNext() {
return !stack.isEmpty();
}

/** @return the next smallest number */
public int next() {
TreeNode smallest = stack.pop();
if (smallest.right != null) {
TreeNode right = smallest.right;
while (right != null) {
stack.push(right);
right = right.left;
}
}
return smallest.val;
}
}

/**
* Your BSTIterator will be called like this:
* BSTIterator i = new BSTIterator(root);
* while (i.hasNext()) v[f()] = i.next();
*/