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

 1     public List<Integer> rightSideView(TreeNode root) {
 2         List<Integer> result = new ArrayList<>();
 3         if (root == null) return result;
 4         Queue<TreeNode> q = new LinkedList<>();
 5         q.add(root);
 6         while (!q.isEmpty())
 7         {
 8             int size = q.size();
 9             TreeNode node = null;
10             for (int i=0;i<size;i++)
11             {
12                 node = q.poll();
13                 if (node.left !=null) q.offer(node.left);
14                 if (node.right !=null) q.offer(node.right);
15             }
16             result.add(node.val);
17         }
18         return result;        
19     }