鏈接：https://leetcode.com/submissions/detail/136407355/
難度：Easy
題目：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.

翻譯：給定一棵二叉樹，找到它的最大深度。最大深度是沿著從根節(jié)點(diǎn)到最遠(yuǎn)葉節(jié)點(diǎn)的最長路徑的節(jié)點(diǎn)的數(shù)量。

思路：用深度優(yōu)先搜索DFS遞歸遍歷二叉樹，搜索判斷左右子樹的深度孰大孰小即可，從根節(jié)點(diǎn)往下一層樹的深度即自增1，遇到NULL時即返回0。

參考代碼：
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)));
    }
}