32_01_PrintTreeFromTopToBottom.py

# -*- coding: utf-8 -*-
# @File    : 32_01_PrintTreeFromTopToBottom.py
# @Date    : 2020-10-31
# @Author  : tc
"""
剑指 Offer 32 - I. 从上到下打印二叉树
从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。

例如:
给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回：

[3,9,20,15,7]


提示：

节点总数 <= 1000

实际就是BFS遍历

"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

import collections
from typing import List

class Solution:
    def levelOrder(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        res, queue = [], collections.deque()
        queue.append(root)
        while queue:
            node = queue.popleft()
            res.append(node.val)
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)
        return res


if __name__ == '__main__':
    pass