LeetCode: BST Iterator

public class BSTIterator {

    private Stack<TreeNode> stack;

    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>();
        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() {
        int result = Integer.MAX_VALUE;
        if (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            result = node.val;
            if (node.right != null) {
                node = node.right;
                while (node != null) {
                    stack.push(node);
                    node = node.left;
                }
            }
        }

        return result;
    }
}