Sum of minimums on all subsegments of an array

#include <bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> arr(n);
    for (int& elem : arr) {
        cin >> elem;
    }

    vector<int> l(n);
    vector<int> stack_of_min;
    stack_of_min.push_back(-1);
    for (int i = 0; i < n; i++) {
        while (stack_of_min.back() != -1 && arr[stack_of_min.back()] >= arr[i]) {
            stack_of_min.pop_back();
        }
        l[i] = stack_of_min.back();
        stack_of_min.push_back(i);
    }

    vector<int> r(n);
    stack_of_min.clear();
    stack_of_min.push_back(n);
    for (int i = n - 1; i >= 0; i--) {
        while (stack_of_min.back() != -1 && arr[stack_of_min.back()] > arr[i]) {
            stack_of_min.pop_back();
        }
        r[i] = stack_of_min.back();
        stack_of_min.push_back(i);
    }

    long long ans = 0;
    for (int i = 0; i < n; i++) {
        ans += 1LL * (i - l[i]) * (r[i] - i) * arr[i];
    }
    cout << ans << endl;
}