// f(0) = 0, f(1) = 1 f(x) = f(x-1) + f(x-2)
int fibonacci(int n) {
    static vector<int> results(31, 0);
    if (n <= 1) return n;
    if (results[n] != 0) return results[n];
    results[n] = fibonacci(n-1) + fibonacci(n-2);
    return results[n];
}