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 node = stack.pop();        int num = node.val;        node = node.right;        while (node != null) {            stack.push(node);            node = node.left;        }        return num;    }}/** * Your BSTIterator will be called like this: * BSTIterator i = new BSTIterator(root); * while (i.hasNext()) v[f()] = i.next(); */