/**
 * 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:

    void dfs(TreeNode* root, int d) {
        if (root->left) dfs(root->left, d+1);
        if (root->right) dfs(root->right, d+1);
        ans = max(ans, d);
    }

    int treeDepth(TreeNode* root) {
        if (root == NULL) return 0;
        ans = 0;
        dfs(root, 1);
        return ans;
    }
private:
    int ans;
};