题目：
题目链接： https://leetcode-cn.com/problems/binary-tree-right-side-view/
解题思路：
树的层序遍历，先放左节点，再放右节点
每次将一层都放完的时候，将最后一个放入的元素加入到结果集中，其余直接清空
然后继续遍历下一层
代码实现：
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def rightSideView(self, root: TreeNode) -> List[int]:
if not root:
return []
stack = [root]
res = []
while stack:
new_stack = []
res.append(stack[-1].val)
for node in stack:
if node.left:
new_stack.append(node.left)
if node.right:
new_stack.append(node.right)
stack = new_stack[:]
return res
