Method update

code/data-structures/fenwick_tree.cpp:4–5

  int n; vi data;
  fenwick_tree(int _n) : n(_n), data(vi(n)) { }
  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;