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.

 1 /**

 2  * Definition for binary tree

 3  * public class TreeNode {

 4  *     int val;

 5  *     TreeNode left;

 6  *     TreeNode right;

 7  *     TreeNode(int x) { val = x; }

 8  * }

 9  */

10 public class Solution {

11     public List<Integer> rightSideView(TreeNode root) {

12         List<Integer> result = new ArrayList<Integer>();

13         if(root == null) return result;

14         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();

15         queue.add(root);

16         queue.add(null);

17         int level = 0;

18         while(queue.size() != 1){

19             TreeNode tmp = queue.poll();

20             if(tmp == null){

21                 queue.add(null);

22                 level ++;

23             }else{

24                 if(result.size() == level){

25                     result.add(tmp.val);

26                 }else{

27                     result.set(level,tmp.val);

28                 }

29                 if(tmp.left != null)queue.add(tmp.left);

30                 if(tmp.right != null)queue.add(tmp.right);

31             }

32         }

33         return result;

34     } 

35 }