/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int treeDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        return 1 +  Math.max(treeDepth(root.left), treeDepth(root.right));
    }
}