LeetCode/cpp/sum-of-subarray-minimums.cpp

#include <vector>

class Solution {
    static const int MOD = 1000000007;

  public:
    int sumSubarrayMins(const std::vector<int> &arr) {
        long s = 0;

        std::vector<int> stack;
        for (auto i = 0; i <= static_cast<int>(arr.size()); ++i) {
            while (!stack.empty() &&
                   (i == arr.size() || arr[stack.back()] >= arr[i])) {
                int mid = stack.back();
                stack.pop_back();

                int l = stack.empty() ? -1 : stack.back();
                int r = i;

                long count = (mid - l) * (r - mid) % MOD;

                s += (count * arr[mid]) % MOD;
                s %= MOD;
            }

            stack.push_back(i);
        }

        return static_cast<int>(s);
    }
};