Method query

code/data-structures/fenwick_tree.cpp:6–9

  void update(int at, int by) {
    while (at < n) data[at] += by, at |= at + 1; }
  int query(int at) {
    int res = 0;
    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 {