链接：https://leetcode.com/submissions/detail/136407355/
难度：Easy
题目：104. Maximum Depth of Binary Tree

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.

翻译：给定一棵二叉树，找到它的最大深度。最大深度是沿着从根节点到最远叶节点的最长路径的节点的数量。

思路：用深度优先搜索DFS递归遍历二叉树，搜索判断左右子树的深度孰大孰小即可，从根节点往下一层树的深度即自增1，遇到NULL时即返回0。

参考代码：
Java

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