Problem p02763 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 58) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

template <typename T = int64> vector<T> make_v(size_t a) {
  return vector<T>(a);
}

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

template <typename F> struct FixPoint : F {
  FixPoint(F &&f) : F(forward<F>(f)) {}

  template <typename... Args> decltype(auto) operator()(Args &&...args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template <typename F> inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}

int main() {
  int N, Q;
  string S;
  cin >> N >> S >> Q;

  int bit[500000] = {};
  for (int i = 0; i < N; i++) {
    bit[i] = S[i] - 'a';
    bit[i] = 1 << bit[i];
  }

  for (int i = 0; i < Q; i++) {
    int t;
    cin >> t;
    if (t == 1) {
      int k;
      char c;
      cin >> k >> c;
      --k;
      bit[k] = c - 'a';
      bit[k] = 1 << bit[k];
    } else {
      int l, r;
      cin >> l >> r;
      --l;
      int tap = 0;
      for (int k = l; k < r; k++)
        tap |= bit[k];
      cout << __builtin_popcount(tap) << endl;
    }
  }
}


Fixed submission:
#include <iomanip>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 58) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v)
    is >> in;
  return is;
}

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}

template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}

template <typename T = int64> vector<T> make_v(size_t a) {
  return vector<T>(a);
}

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

template <typename F> struct FixPoint : F {
  FixPoint(F &&f) : F(forward<F>(f)) {}

  template <typename... Args> decltype(auto) operator()(Args &&...args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template <typename F> inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}

int main() {
  int N, Q;
  string S;
  cin >> N >> S >> Q;

  int bit[500000] = {};
  for (int i = 0; i < N; i++) {
    bit[i] = S[i] - 'a';
    bit[i] = 1 << bit[i];
  }

  for (int i = 0; i < Q; i++) {
    int t;
    cin >> t;
    if (t == 1) {
      int k;
      char c;
      cin >> k >> c;
      --k;
      bit[k] = c - 'a';
      bit[k] = 1 << bit[k];
    } else {
      int l, r;
      cin >> l >> r;
      --l;
      int tap = 0;
      for (int k = l; k < r; k++)
        tap |= bit[k];
      cout << __builtin_popcount(tap) << endl;
    }
  }
}