人狠话不多，直接上代码

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int max = 0;
    int cur = 0;
    int maxDepth(TreeNode* root) {
        // write code here
        if(!root)return cur;
        cur++;
        if(max < cur) max = cur;
        maxDepth(root->left);
        maxDepth(root->right);
        cur--;
        return max;
    }
};