[Leet Code] Find Largest Value in Each Tree Row

Input: root = [1,3,2,5,3,null,9]
Output: [1,3,9]
Input: root = [1,2,3]
Output: [1,3]
Input: root = [1]
Output: [1]
Input: root = [1,null,2]
Output: [1,2]
Input: root = []
Output: []
# 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)

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