Binary Tree Right Side 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 }