LeetCode199. 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].

分析:这道题可以用广度优先搜索也可以用深度优先搜索。这里用广度优先方法解决:
如果root为空,则返回空vector。建立存放TreeNode指针的队列,将root结点入队;出队root的同时入队root的存在的left和right结点;按照层序遍历的方式,把每一层的最后一个结点的值存入vector中,最后返回vector~