Count ways to build street under given constraints

#include <iostream>
using namespace std;

// A recursive function to find nth catalan number
unsigned long int catalan(unsigned int n)
{
	// Base case
	if (n <= 1)
		return 1;

	// catalan(n) is sum of
	// catalan(i)*catalan(n-i-1)
	unsigned long int res = 0;
	for (int i = 0; i < n; i++)
		res += catalan(i)
			* catalan(n - i - 1);

	return res;
}

// Driver code
int main()
{
	for (int i = 0; i < 10; i++)
		cout << catalan(i) << " ";
	return 0;
}