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].

题目含义：从右面观察一颗二叉树，输出由上到下打印的节点

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