leetcodeday515–在每个树行中找最大值

给定一棵二叉树的根节点 root ，请找出该二叉树中每一层的最大值。

示例1：

输入: root = [1,3,2,5,3,null,9]
输出: [1,3,9]

示例2：

输入: root = [1,2,3]
输出: [1,3]

# @lc app=leetcode.cn id=515 lang=python3
#
# [515] 在每个树行中找最大值
#

# @lc code=start
# 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: Optional[TreeNode]) -> List[int]:
        if not root: return [] #注意特殊情况：树为空返回[]
        queue = [root]
        list1 = []
        while queue:
            list2 = -2**31
            for i in range(len(queue)):
                a = queue.pop(0)#元素出队列
                if a.left:
                    queue.append(a.left)
                if a.right:
                    queue.append(a.right)
                list2=max(list2,a.val)
            list1.append(list2)
        return list1
# @lc code=end