// Function to return the maximum product of a subarray of a given array
ll max_prod_subarr(vll &v) {
    int n = sz(v);
    if(n == 0) return 0;

    // to store the final result
    ll res = v[0];

    // max_ending_here = maximum product ending at the current index.
    // min_ending_here = minimum product ending at the current index.
    ll max_ending_here = v[0];
    ll min_ending_here = v[0];

    for(int i = 1; i < n; i++) {
        // update the maximum product ending at the current index
        max_ending_here = max(v[i], v[i] * max_ending_here);

        // update the minimum product ending at the current index
        min_ending_here = min(v[i], v[i] * min_ending_here);

        // swap max_ending_here & min_ending_here if required
        if(max_ending_here < min_ending_here) {
            swap(max_ending_here, min_ending_here);
        }

        // update final result
        res = max(res, max_ending_here);
    }

    return res;
}