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

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int calculateDepth(TreeNode* root) 
    {
        // 采用一种不借助全局变量记录深度的递归方法
        if(root == nullptr)
        {
            return 0;
        }

        int left_depth = calculateDepth(root->left); // 定义为int型，返回每一次递归的深度
        int right_depth = calculateDepth(root->right);

        return max(left_depth,right_depth) + 1;
    }
};