Find X-Sum of All K-Long Subarrays I — C++

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

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

// freq table, ordered set, two pointers, sliding window
class Solution {
public:
    vector<int> findXSum(vector<int>& nums, int k, int x) {
        using ordered_set = tree<pair<int, int>, null_type, greater<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update>;
        ordered_set os;
        unordered_map<int, int> cnt;
        const auto& update = [&](int v, int d, int curr) {
            if (d == 1) {
                os.insert({cnt[v], v});
            }

            if (os.order_of_key(pair(cnt[v], v)) < x) {
                curr += d * cnt[v] * v;
                if (x < size(os)) {
                    const auto [nc, nv] = *(os.find_by_order(x));
                    curr -= d * nc * nv;
                }
            }
            if (d != 1) {
                os.erase(pair(cnt[v], v));
            }
            return curr;
        };

        vector<int> result;
        for (int i = 0, curr = 0; i < size(nums); ++i) {
            if (cnt.count(nums[i])) {
                curr = update(nums[i], -1, curr);
            }
            ++cnt[nums[i]];
            curr = update(nums[i], +1, curr);
            if (i < k - 1) {
                continue;
            }
            result.emplace_back(curr);
            curr = update(nums[i - (k - 1)], -1, curr);
            --cnt[nums[i - (k - 1)]];
            if (cnt[nums[i - (k - 1)]]) {
                curr = update(nums[i - (k - 1)], +1, curr);
            } else {
                cnt.erase(nums[i - (k - 1)]);
            }
        }
        return result;
    }
};