[LeetCode 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.
For example: Given the following binary tree,

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

You should return  [1, 3, 4] .
Solution: 
1. use queue to do level iteration.

public List<Integer> rightSideView(TreeNode root) {
        List<Integer> result = new LinkedList<>();
        if(root == null) return result;
        Queue<TreeNode> levelNodes = new LinkedList<>();
        levelNodes.offer(root);
        while(levelNodes.size()>0){
            TreeNode n2 = levelNodes.peek();
            int len = levelNodes.size();
            for(int i=0;i<len;i++){
                TreeNode n1 = levelNodes.poll();
                if(n1.right!=null)
                    levelNodes.offer(n1.right);
                if(n1.left!=null)
                    levelNodes.offer(n1.left);
                
            }
            result.add(n2.val);
        }
        return result;
    }