/**
 * 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 ans = 0;
    int countNodes(TreeNode* root) {
        if (!root)  return ans;
        ans += 1;
        dfs(root);
        return ans;
    }
    void dfs(TreeNode* root) {
        if (root->left) {
            ans ++;
            dfs(root->left);
        }
        if (root->right) {
            ans ++;
            dfs(root->right);
        }
    }
};