Just a bit shorter/different than previous solutions.

Ruby:

```
def max_depth(root)
root ? 1 + [max_depth(root.left), max_depth(root.right)].max : 0
end
```

Python:

```
def maxDepth(self, root):
return 1 + max(map(self.maxDepth, (root.left, root.right))) if root else 0
```