c++ – C++17 Recursive Fibonacci calculation with memoization

Compiler is g++ 4.2. I’m new to C++, but I’ve done a lot of data science, web scraping, and some socketing stuff in Python. This code generates the nth Fibonacci number, either with a naive implementation or with caching.

#include <iostream>
#include <string>
#include <unordered_map>
#define BIG unsigned long long int

std::unordered_map<int, BIG> umap;


int fib(int n) {
    if (n < 2) {
        return n;
    }
    return fib(n-1) + fib(n-2);
}

BIG fib_with_memo(int n) {
    if (umap.find(n) == umap.end()) { // if F_n not in cache, calculate, cache, and return it
        BIG val = fib_with_memo(n-1) + fib_with_memo(n-2);
        umap(n) = val;
        return val;
    }
    return umap(n); // otherwise return the cached value
}

int main(int argc, char** argv) {
    umap(0) = 0;
    umap(1) = 1;

    int input = std::stoi( std::string(argv(1)) );
    
    std::cout << fib_with_memo(input) << std::endl;
    return 0;
}

I used a std::unordered_map (hash table) because its lookup/insertion times are $O(1)$ (compared to the std::map, which has $O(log(n))$ for both.)