<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:

        if not root:
            return 0

        # BFS
        # queue = deque()

        # queue.append(root)

        # depth = 0

        # while queue:
        #     size = len(queue)
        #     while size > 0:
        #         curr = queue.popleft()
        #         if curr.left:
        #             queue.append(curr.left)
        #         if curr.right:
        #             queue.append(curr.right)
        #         size -= 1
        #     depth += 1

        # return depth


        # Recursive DFS
        # left_depth = self.maxDepth(root.left)
        # right_depth = self.maxDepth(root.right)

        # return 1 + max(left_depth, right_depth)

        # DFS - stack

        stack = [(root, 1)]
        maxDepth = 0

        while stack:
            curr, depth = stack.pop()

            if not curr:
                continue

            maxDepth = max(maxDepth, depth)
            stack.append((curr.right, depth+1))
            stack.append((curr.left, depth+1))
        
        return maxDepth
17300