515. Find Largest Value in Each Tree Row

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def largestValues(self, root):
        if not root:
            return []
        list1=[root,None]
        ans=[]
        l=2
        mx=root.val
        while l>1:
            if list1[0]==None:
                ans.append(mx)
                mx=-float('inf')
                list1.pop(0)
                list1.append(None)
            
            x=list1.pop(0)
            l-=1
            if mx<x.val:
                mx=x.val
            if x.left:
                list1.append(x.left)
                l+=1
            if x.right:
                list1.append(x.right)
                l+=1
        ans.append(mx)
        return ans