199.Binary Tree Right Side View

Given a binary tree,imagine yourself standing on the?right?side of it,return the values of the nodes you can see ordered from top to bottom.
Example:
Input:?[1,2,3,null,5,4]
Output:?[1,4]
Explanation:

   1            <---
 /   2     3         <---
        5     4       <---



/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
      Queue<TreeNode> queue = new LinkedList<>();
      List<Integer> result = new ArrayList<>();
      if(root == null) return result;
      queue.offer(root);
      while (!queue.isEmpty()){
        int size = queue.size();
        for (int i = 0; i < size; i++){
          TreeNode cur = queue.poll();
          if(i == 0){
            result.add(cur.val);
          }
          if(cur.right != null) queue.offer(cur.right);
          if(cur.left != null) queue.offer(cur.left);
        }
      }
      return result;
    }
}