<p>Given the <code>root</code> of a binary tree, return <em>its maximum depth</em>.</p>

<p>A binary tree&#39;s <strong>maximum depth</strong>&nbsp;is the number of nodes along the longest path from the root node down to the farthest leaf node.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/26/tmp-tree.jpg" style="width: 400px; height: 277px;" />
<pre>
<strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> 3
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1,null,2]
<strong>Output:</strong> 2
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 10<sup>4</sup>]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        #DFS solution - Depth First Solution
        stack = [[root,1]]
        res = 0

        while stack:
            node,depth = stack.pop()
            if node:
                res = max(res,depth)
                stack.append([node.left, depth + 1])
                stack.append([node.right, depth + 1])
        return res

        # #BFS solution - Breath first - check each level. use queues
        # depth = 0
        # if root is None:
        #     return depth
        # q = deque([root])
        # while q:
        #     for i in range(len(q)):
        #         node = q.popleft()
        #         if node.left:
        #             q.append(node.left)
        #         if node.right:
        #             q.append(node.right)
        #     depth += 1
        
        # return depth
        # Recursive solution
        # depth = 0
        # if root is None:
        #     return depth
        # depth = 1 + max(self.maxDepth(root.left),self.maxDepth(root.right))
        # return depth
        # # check_node = check_node.
        # # print(Check_node.val)
17300