剑指 Offer 55 - I. 二叉树的深度

分析：
普通递归调用

/**
 * 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;
        }
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        return left > right ? left+1 :right+1;
    }
}

题目：https://leetcode-cn.com/problems/er-cha-shu-de-shen-du-lcof/