[Leet Code] Find Largest Value in Each Tree Row

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

Problem:

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: []

Solution:

# 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):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root:
            elements = [[root.val]]
            stack = [root]
            self.helper(stack, elements)
            for i in range(len(elements)):
                elements[i] = max(elements[i])
                
            return elements
        else:
            return root
            
    def helper(self, stack, elements):
        while len(stack) > 0:
            values = []
            length = len(stack)
            for i in range(length):
                node = stack.pop(0)
                if node.left:
                    stack.append(node.left)
                    values.append(node.left.val)
                if node.right:
                    stack.append(node.right)
                    values.append(node.right.val)
            if values != []:
                elements.append(values)