199. Binary Tree Right Side View

class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
       helper(root, ans, 0);
        return ans;
    }
    public void helper(TreeNode root , List<Integer> ans, int level){
        if(root == null) return;
        if(level == ans.size()){
            ans.add(root.val);
        }
        helper(root.right, ans, level + 1);
        helper(root.left, ans, level + 1);
    }
}