/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int treeDepth(TreeNode* root) {
        if (!root) return 0;
        return max(treeDepth(root->left), treeDepth(root->right)) + 1;  //+1表示根节点
    }
};