算法模板目录 ：个人算法模板

template<typename T>
struct fenwick2D{
    vector<vector<T>> tr;
    int n, m;
    fenwick2D(int _n, int _m) : n(_n), m(_m) {
        tr.resize(n + 1);
        for (int i = 0; i <= n; i++) {
            tr.resize(m + 1);
        }
    }
    void add(int x, int y, int val){ // 1 <= x <= n, 1 <= y <= m
        for(int i = x; i <= n; i += i & -i) {
            for(int j = y; j <= m; j += j & -j) {
                tr[i][j] += val;
            }
        }
    }
    T Sum(int x, int y) { // 1 <= x <= n, 1 <= y <= m
        T res = 0;
        for(int i = x; i > 0; i -= i & -i) {
            for(int j = y; j > 0; j -= j & -j) {
                res += tr[i][j];
            }
        }
        return res;
    }
    T Sum(int x1, int y1, int x2, int y2) {  // sum[x1..x2, y1..y2]
        return Sum(x2, y2) - Sum(x2, y1 - 1) - Sum(x1 - 1, y2) + Sum(x1 - 1, y1 - 1);
    }
};