[Leet Code] Maximum Depth Of Binary Tree

Leetcode: https://leetcode.com/problems/maximum-depth-of-binary-tree/

Problem:

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

Example 3:

Input: root = []
Output: 0

Example 4:

Input: root = [0]
Output: 1

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