Maximum Sum Obtained of Any Permutation — C++

// Time:  O(nlogn)
// Space: O(n)

class Solution {
public:
    int maxSumRangeQuery(vector<int>& nums, vector<vector<int>>& requests) {
        static const int MOD = 1e9 + 7;

        vector<int> count(size(nums));
        for (const auto& req : requests) {
            ++count[req[0]];
            if (req[1] + 1 < size(count)) {
                --count[req[1] + 1];
            }
        }
        partial_sum(begin(count), end(count), begin(count));
        sort(begin(nums), end(nums));
        sort(begin(count), end(count));
        int result = 0;
        for (int i = 0; i < size(nums); ++i) {
            result = addmod(result, mulmod(nums[i], count[i], MOD), MOD);
        }
        return result;
    }

private:
    uint32_t addmod(uint32_t a, uint32_t b, uint32_t mod) {  // avoid overflow
        a %= mod, b %= mod;
        if (mod - a <= b) {
            b -= mod;  // relied on unsigned integer overflow in order to give the expected results
        }
        return a + b;
    }

    // reference: https://stackoverflow.com/questions/12168348/ways-to-do-modulo-multiplication-with-primitive-types
    uint32_t mulmod(uint32_t a, uint32_t b, uint32_t mod)  {  // avoid overflow
        a %= mod, b %= mod;
        uint32_t result = 0;
        if (a < b) {
            swap(a, b);
        }
        while (b > 0)  { 
            if (b % 2 == 1) {
                result = addmod(result, a, mod);
            }
            a = addmod(a, a, mod);
            b /= 2; 
        } 
        return result; 
    }
};