OK, easiest one yet.

Problem: Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

```
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int maxDepth(TreeNode root) {
if (root == null) return 0;
int heightLeft = maxDepth(root.left) + 1;
int heightRight = maxDepth(root.right) + 1;
return Math.max(heightLeft, heightRight);
}
}
```

```
```