Alternating Groups III — C++

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

// sorted list, freq table, bit, fenwick tree
class Solution {
private:
    class BIT {
    public:
        BIT(int n) : bit_(n + 1) {  // 0-indexed
        }
        
        void add(int i, int val) {
            ++i;
            for (; i < size(bit_); i += lower_bit(i)) {
                bit_[i] += val;
            }
        }

        int query(int i) const {
            ++i;
            int total = 0;
            for (; i > 0; i -= lower_bit(i)) {
                total += bit_[i];
            }
            return total;
        }
    
    private:
        int lower_bit(int i) const {
            return i & -i;
        }
        
        vector<int> bit_;
    };

public:
    vector<int> numberOfAlternatingGroups(vector<int>& colors, vector<vector<int>>& queries) {
        const int n = size(colors);
        set<int> bst;
        BIT bit1(n + 1), bit2(n + 1);
        const auto& update = [&](int i, int d) {
            if (d == +1) {
                bst.emplace(i);
                if (size(bst) == 1) {
                    bit1.add(n, +1);
                    bit2.add(n, +n);
                }
            }
            auto curr = bst.find(i);
            auto prv = prev(curr != begin(bst) ? curr : end(bst));
            auto nxt = next(curr);
            if (nxt == end(bst)) {
                nxt = begin(bst);
            }
            if (size(bst) != 1) {
                int l = ((*nxt) - (*prv) + (n - 1)) % n + 1;
                bit1.add(l, d * -1);
                bit2.add(l, d * -l);
                l = ((*curr) - (*prv) + n) % n;
                bit1.add(l, d * +1);
                bit2.add(l, d * +l);
                l = ((*nxt) - (*curr) + n) % n;
                bit1.add(l, d * +1);
                bit2.add(l, d * +l);
            }
            if (d == -1) {
                if (size(bst) == 1) {
                    bit1.add(n, -1);
                    bit2.add(n, -n);
                }
                bst.erase(curr);
            }
        };

        vector<int> result;
        for (int i = 0; i < n; ++i) {
            if (colors[i] == colors[(i + 1) % n]) {
                update(i, +1);
            }
        }
        for (const auto &q : queries) {
            if (q[0] == 1) {
                const int l = q[1];
                result.emplace_back(!empty(bst) ? (bit2.query(n) - bit2.query(l - 1)) - 
                                                  (l - 1) * (bit1.query(n) - bit1.query(l - 1)) : n);
                continue;
            }
            const int i = q[1];
            if (colors[i] == q[2]) {
                continue;
            }
            colors[i] = q[2];
            update((i - 1 + n) % n, colors[i] == colors[(i - 1 + n) % n] ? +1 : -1);
            update(i, colors[i] == colors[(i + 1) % n] ? +1 : -1);
        }
        return result;
    }
};