LeetCode <dfs>173.Binary Search Tree Iterator

public class BSTIterator {
     Stack<TreeNode> stack;
    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>(); // initialize the stack
        stackHelper(root);
    }

    public void stackHelper(TreeNode root){
        if (root == null) return;
        stackHelper(root.right); 
        stack.push(root);
        stackHelper(root.left);      
    }

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

    /** @return the next smallest number */
    public int next() {
        return stack.pop().val;
    }
}