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]

其實就是層序遍歷 的每層的最後一個元素！！！！

 1 class Solution {
 2     
 3     public List<Integer> rightSideView(TreeNode root) {
 4         List<Integer> res = new ArrayList<Integer>();
 5         Queue<TreeNode> queue = new LinkedList<TreeNode>();
 6         if(root==null) return res;
 
 7         queue.offer(root);
 8         int level_num = 1;
 9         while (!queue.isEmpty()) {
10             level_num = queue.size();
11             for(int i = 0; i < level_num; i++){
12                 TreeNode node = queue.poll();
13                 if(i==level_num-1)
14                     res.add(node.val);
 
15                 if(node.left != null) queue.offer(node.left);
16                 if(node.right != null) queue.offer(node.right);
17                 
18             }
19         }
20         return res;
21     }
22 }

199. Binary Tree Right Side View -----層序遍歷