Leetcode 515. Find Largest Value in Each Tree Row

# 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]:
        if not root: return 
        q = collections.deque([root, None])
        
        keep = []
        res = []
        
        while q:
            node = q.popleft()
            
            if node is None:
                if q: q.append(None) # prevent infinite loop
                res.append(max(keep))
                keep = []
            else:    
                keep.append(node.val)

                if node.left: q.append(node.left)
                if node.right: q.append(node.right)
        
        return res

https://leetcode.com/problems/find-largest-value-in-each-tree-row/