Problem p02763 (C++)

Buggy submission (status: Runtime Error):
/**
 *    author:  zjsdut
 *    created: 2020/03/29 21:58:23
 **/
#include <bits/stdc++.h>

using namespace std;

struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
  };
} fast_ios_;

template <typename A, typename B> bool chkmin(A &a, const B &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

template <typename A, typename B> bool chkmax(A &a, const B &b) {
  if (b > a) {
    a = b;
    return true;
  }
  return false;
}

template <typename T> istream &operator>>(istream &stream, vector<T> &vec) {
  for (auto &x : vec)
    stream >> x;
  return stream;
}

void scan() {}

template <class T, class... Args> void scan(T &a, Args &...rest) {
  cin >> a;
  scan(rest...);
}

using ll = long long;
using vl = vector<ll>;
using vb = vector<bool>;
using vi = vector<int>;
using pii = pair<int, int>;
template <typename T> using vv = vector<vector<T>>;
#define rng(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define up(i, a, b) for (int i = int(a); i <= int(b); ++i)
#define down(i, b, a) for (int i = int(b); i >= int(a); i--)
#define TOKENPASTE(x, y) x##y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)
#define rep(n)                                                                 \
  for (int TOKENPASTE2(_iter_, __LINE__) = 0,                                  \
                               TOKENPASTE2(_num_, __LINE__) = (int)(n);        \
       TOKENPASTE2(_iter_, __LINE__) < TOKENPASTE2(_num_, __LINE__);           \
       ++TOKENPASTE2(_iter_, __LINE__))
#define FOR(x, cont) for (const auto &x : cont)
#define For(x, cont) for (auto &x : cont)
#define all(x) begin(x), end(x)
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ep emplace
#define SZ(x) (int)(x).size()
#define UNIQ(vec) (vec).erase(unique(all(vec)), end(vec))
#define LB(cont, x) int(lower_bound(all(cont), x) - begin(cont))
#define UB(cont, x) int(upper_bound(all(cont), x) - begin(cont))

template <typename T>
ostream &operator<<(ostream &stream, const vector<T> &vec) {
  if (!vec.empty()) {
    stream << vec[0];
    for (size_t i = 1; i != vec.size(); ++i)
      stream << ' ' << vec[i];
  }
  return stream;
}

template <typename T> void print(const vector<T> &t) { cout << t << '\n'; }

template <typename T> void print(const vector<vector<T>> &t) {
  for (const auto &row : t) {
    print(row);
  }
}

template <typename T> void print(const T &t) { cout << t << ' '; }

template <typename T, typename... Args>
void print(const T &t, const Args &...rest) {
  print(t);
  print(rest...);
}

template <typename T> void println(const T &t) { cout << t << '\n'; }

template <typename T, typename... Args>
void println(const T &t, const Args &...rest) {
  print(t);
  println(rest...);
}
template <typename T> class BIT {
  // 0-indexed
  vector<T> a;
  using BinOp = function<T(T, T)>;
  BinOp bin_op;
  const T init;

public:
  explicit BIT(
      int n, BinOp op = [](T x, T y) { return x + y; }, T init = T{})
      : bin_op(op), init(init), a(n, init) {}

  T prefix(int x) const {
    auto res = init;
    while (x >= 0) {
      res = bin_op(a[x], res);
      x = (x & (x + 1)) - 1;
    }
    return res;
  }

  void modify(int x, T v) {
    while (x < SZ(a)) {
      a[x] = bin_op(a[x], v);
      x |= (x + 1);
    }
  }

  void clear() { fill(a.begin(), a.end(), init); }
};
// debug code by tourist
string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#if defined LOCAL && !defined DUIPAI
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

int main() {
#if defined LOCAL && !defined DUIPAI
  ifstream in("in.txt");
  cin.rdbuf(in.rdbuf());
#endif
  int n;
  string s;
  scan(n, s);
  vector<BIT<int>> a(26, BIT<int>(n));
  rng(i, 0, n) { a[s[i] - 'a'].modify(i, 1); }
  int q;
  scan(q);
  rep(q) {
    int t;
    scan(t);
    if (t == 1) {
      int i;
      char c;
      scan(i, c);
      --i;
      a[c - 'a'].modify(i, 1);
      a[s[i] - 'a'].modify(i, -1);
      s[i] = c;
      debug(s);
    } else {
      int l, r;
      scan(l, r);
      --l, --r;
      int ans = 0;
      debug(s);
      rng(i, 0, n) { ans += a[i].prefix(r) - a[i].prefix(l - 1) > 0; }
      println(ans);
    }
  }
  return 0;
}

Fixed submission:
/**
 *    author:  zjsdut
 *    created: 2020/03/29 21:58:23
 **/
#include <bits/stdc++.h>

using namespace std;

struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
  };
} fast_ios_;

template <typename A, typename B> bool chkmin(A &a, const B &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

template <typename A, typename B> bool chkmax(A &a, const B &b) {
  if (b > a) {
    a = b;
    return true;
  }
  return false;
}

template <typename T> istream &operator>>(istream &stream, vector<T> &vec) {
  for (auto &x : vec)
    stream >> x;
  return stream;
}

void scan() {}

template <class T, class... Args> void scan(T &a, Args &...rest) {
  cin >> a;
  scan(rest...);
}

using ll = long long;
using vl = vector<ll>;
using vb = vector<bool>;
using vi = vector<int>;
using pii = pair<int, int>;
template <typename T> using vv = vector<vector<T>>;
#define rng(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define up(i, a, b) for (int i = int(a); i <= int(b); ++i)
#define down(i, b, a) for (int i = int(b); i >= int(a); i--)
#define TOKENPASTE(x, y) x##y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)
#define rep(n)                                                                 \
  for (int TOKENPASTE2(_iter_, __LINE__) = 0,                                  \
                               TOKENPASTE2(_num_, __LINE__) = (int)(n);        \
       TOKENPASTE2(_iter_, __LINE__) < TOKENPASTE2(_num_, __LINE__);           \
       ++TOKENPASTE2(_iter_, __LINE__))
#define FOR(x, cont) for (const auto &x : cont)
#define For(x, cont) for (auto &x : cont)
#define all(x) begin(x), end(x)
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ep emplace
#define SZ(x) (int)(x).size()
#define UNIQ(vec) (vec).erase(unique(all(vec)), end(vec))
#define LB(cont, x) int(lower_bound(all(cont), x) - begin(cont))
#define UB(cont, x) int(upper_bound(all(cont), x) - begin(cont))

template <typename T>
ostream &operator<<(ostream &stream, const vector<T> &vec) {
  if (!vec.empty()) {
    stream << vec[0];
    for (size_t i = 1; i != vec.size(); ++i)
      stream << ' ' << vec[i];
  }
  return stream;
}

template <typename T> void print(const vector<T> &t) { cout << t << '\n'; }

template <typename T> void print(const vector<vector<T>> &t) {
  for (const auto &row : t) {
    print(row);
  }
}

template <typename T> void print(const T &t) { cout << t << ' '; }

template <typename T, typename... Args>
void print(const T &t, const Args &...rest) {
  print(t);
  print(rest...);
}

template <typename T> void println(const T &t) { cout << t << '\n'; }

template <typename T, typename... Args>
void println(const T &t, const Args &...rest) {
  print(t);
  println(rest...);
}
template <typename T> class BIT {
  // 0-indexed
  vector<T> a;
  using BinOp = function<T(T, T)>;
  BinOp bin_op;
  const T init;

public:
  explicit BIT(
      int n, BinOp op = [](T x, T y) { return x + y; }, T init = T{})
      : bin_op(op), init(init), a(n, init) {}

  T prefix(int x) const {
    auto res = init;
    while (x >= 0) {
      res = bin_op(a[x], res);
      x = (x & (x + 1)) - 1;
    }
    return res;
  }

  void modify(int x, T v) {
    while (x < SZ(a)) {
      a[x] = bin_op(a[x], v);
      x |= (x + 1);
    }
  }

  void clear() { fill(a.begin(), a.end(), init); }
};
// debug code by tourist
string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#if defined LOCAL && !defined DUIPAI
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

int main() {
#if defined LOCAL && !defined DUIPAI
  ifstream in("in.txt");
  cin.rdbuf(in.rdbuf());
#endif
  int n;
  string s;
  scan(n, s);
  vector<BIT<int>> a(26, BIT<int>(n));
  rng(i, 0, n) { a[s[i] - 'a'].modify(i, 1); }
  int q;
  scan(q);
  rep(q) {
    int t;
    scan(t);
    if (t == 1) {
      int i;
      char c;
      scan(i, c);
      --i;
      a[c - 'a'].modify(i, 1);
      a[s[i] - 'a'].modify(i, -1);
      s[i] = c;
      debug(s);
    } else {
      int l, r;
      scan(l, r);
      --l, --r;
      int ans = 0;
      debug(s);
      rng(i, 0, 26) { ans += a[i].prefix(r) - a[i].prefix(l - 1) > 0; }
      println(ans);
    }
  }
  return 0;
}