We use a recursive DFS approach. The depth of a node is 1 + max(depth of left child, depth of right child).

1. If root is None, return 0.
2. Calculate left_depth = max_depth(root.left).
3. Calculate right_depth = max_depth(root.right).
4. Return max(left_depth, right_depth) + 1.

• Time Complexity: O(N).
• Space Complexity: O(H).

def max_depth(root):
    if not root:
        return 0
    return 1 + max(max_depth(root.left), max_depth(root.right))