515. 在每个树行中找最大值

# 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 root is None:
            return []

        nodes, vals = deque([root, '#']), [[]]
        while nodes:
            node = nodes.popleft()
            if node == '#':
                if nodes:
                    vals.append([])
                    nodes.append('#')
                continue

            vals[-1].append(node.val)

            if node.left:
                nodes.append(node.left)
            if node.right:
                nodes.append(node.right)

        return list(map(max, vals))