## LeetCode | 0515. 在每个树行中找最大值【Python】

Wonz 2021-01-20 22:45:19
Python github def

## Problem

LeetCode

Given the `root` of a binary tree, return an array of the largest value in each row of the tree (0-indexed).

Example 1:

``````Input: root = [1,3,2,5,3,null,9]
Output: [1,3,9]
``````

Example 2:

``````Input: root = [1,2,3]
Output: [1,3]
``````

Example 3:

``````Input: root = [1]
Output: [1]
``````

Example 4:

``````Input: root = [1,null,2]
Output: [1,2]
``````

Example 5:

``````Input: root = []
Output: []
``````

Constraints:

• The number of nodes in the tree will be in the range `[0, 104]`.
• `-231 <= Node.val <= 231 - 1`

## 问题

``````输入:
1
/ \
3 2
/ \ \
5 3 9

``````

## 思路

BFS

``````使用 BFS 遍历完每一层，将该层最大值加入 res 中。
``````

### Python3 代码

``````# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def largestValues(self, root: TreeNode) -> List[int]:
import collections
if not root:
return []
res = []
q = collections.deque()
q.append(root)
while q:
size = len(q)
tmp_max = -float('inf')
# 取每一层最大值
for i in range(size):
node = q.popleft()
tmp_max = max(tmp_max, node.val)
# 一层树遍历完，加入该层最大值
if i == size - 1:
res.append(tmp_max)
if node.left:
q.append(node.left)
if node.right:
q.append(node.right)
return res
``````

## GitHub 链接

Python

https://my.oschina.net/wonz/blog/4916959