二叉树的右视图

https://leetcode-cn.com/problems/binary-tree-right-side-view/submissions/
给定一棵二叉树，想象自己站在它的右侧，按照从顶部到底部的顺序，返回从右侧所能看到的节点值。

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