# [Leet Code] Maximum Depth Of Binary Tree

`Input: root = [3,9,20,null,null,15,7]Output: 3`
`Input: root = [1,null,2]Output: 2`
`Input: root = []Output: 0`
`Input: root = [0]Output: 1`
`# 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 = rightclass 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)`

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