Method rsq

code/data-structures/fenwick_tree.cpp:10–10

    while (at >= 0) res += data[at], at = (at & (at + 1)) - 1;
    return res; }
  int rsq(int a, int b) { return query(b) - query(a - 1); }
};
struct fenwick_tree_sq {
  int n; fenwick_tree x1, x0;