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].

分析：

这题有迷惑性，不要以为只是让你找出最右边的节点，如果左边节点比右边节点高，那么左边节点的右边部分也要输出来。

这题可以按层获取树的节点，然后把每层最右边的找出来。

 1 /**
 2  * Definition for a binary tree node.
 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<>();
13         if (root == null) return result;
14 
15         Queue<TreeNode> queue = new LinkedList<>();
16         queue.offer(root);
17 
18         while (!queue.isEmpty()) {
19             int size = queue.size();
20             for (int i = 0; i < size; i++) {
21                 TreeNode top = queue.poll();
22                 if (i == size - 1) {
23                     result.add(top.val);
24                 }
25                 if (top.left != null) {
26                     queue.add(top.left);
27                 }
28                 if (top.right != null) {
29                     queue.add(top.right);
30                 }
31             }
32         }
33         return result;
34     }
35 }