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));
}
}