Leetcode 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].

BFS 每层都以从右向左的顺序扔数组里取出第一个到结果数组中，然后再对下一层同样操作。

class Solution(object):
    def rightSideView(self, root):
        if not root:
            return []
        a, ans = [root], []
        while a:
            b = []
            ans.append(a[0].val)
            for node in a:
                if node.right:
                    b.append(node.right)
                if node.left:
                    b.append(node.left)
            a = b
        return ans