Gray codes in c++

gray.cc

#include <vector>
#include <string>
#include <iostream>

using namespace std;

void gray(int n, vector<string>& code) {
	if (n == 1) {
		code.push_back(string("0"));
		code.push_back(string("1"));
	} else {
		vector<string> prev;
		gray(n - 1, prev);
		for (size_t i = 0; i < prev.size(); ++i) {
			code.push_back(string("0") + prev[i]);
		}
		/*
		 * The first version of this program had a subtle bug in it.
		 * Can you find it? Hint: will this loop ever terminate?
		 *
		 *      for (size_t i = prev.size() - 1; i >= 0; --i) {
		 */
		for (long i = long(prev.size() - 1); i >= 0; --i) {
			code.push_back(string("1") + prev[i]);
		}
	}
}

int main(int argc, char** argv) {
	vector<string> code;
	gray(8, code);
	cout << "[";
	for (size_t i = 0; i < code.size(); ++i) {
		cout << code[i];
		if (i != code.size() - 1) {
			cout << ",";
		}
	}
	cout << "]";
	return 0;
}