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.

For example:
Given the following binary tree,

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

You should return [1, 3, 4].

Top view, Bot view, right view, and left view.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        if(root == null) return result;
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        queue.add(null);
        int level = 0;
        while(queue.size() != 1){
            TreeNode tmp = queue.poll();
            if(tmp == null){
                queue.add(null);
                level ++;
            }else{
                if(result.size() == level){
                    result.add(tmp.val);
                }else{
                    result.set(level,tmp.val);
                }
                if(tmp.left != null)queue.add(tmp.left);
                if(tmp.right != null)queue.add(tmp.right);
            }
        }
        return result;
    } 
}