Count complete tree Nodes

class Solution {
public:
    void f(TreeNode* root, int &count){

        if(root==NULL)return;

        f(root->left,count);
        count++;
        f(root->right,count);

        return;

    }
    int countNodes(TreeNode* root) {
        int count = 0;
        f(root,count);

        return count;
    }
};