Simple java solution, recursion

import java.util.*;
import java.lang.Math;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    public int maxDepth (TreeNode root) {
        // write code here
        if (root == null){return 0;}
        return 1+Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}