leetcode 199 binary tree right side view

 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class  {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        helper(ans, root, 0);
        return ans;
    }
    public void helper(List<Integer> ans, TreeNode root, int level) {
        if (root == null)
            return ;
        if (ans.size() == level) {
            ans.add(root.val);
        }   
        if (root.right != null)
            helper(ans, root.right, level+1);
        if (root.left != null)
            helper(ans, root.left, level+1);
    }
}