Problem p02763 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <complex>
#include <cstdint>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <stdio.h>
#include <string>
#include <time.h>
#include <utility>
#include <vector>
using namespace std;
using pii = pair<int, int>;
using ll = long long;
using ld = long double;
#define pb push_back
#define mp make_pair
#define sc second
#define fr first
#define stpr setprecision
#define cYES cout << "YES" << endl
#define cNO cout << "NO" << endl
#define cYes cout << "Yes" << endl
#define cNo cout << "No" << endl
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define Rep(i, a, b) for (ll i = (a); i < (b); ++i)
#define rrep(i, n) for (ll i = n - 1; i >= 0; i--)
#define rRep(i, a, b) for (ll i = a; i >= b; i--)
#define crep(i) for (char i = 'a'; i <= 'z'; ++i)
#define psortsecond(A, N)                                                      \
  sort(A, A + N,                                                               \
       [](const pii &a, const pii &b) { return a.second < b.second; });
#define ALL(x) (x).begin(), (x).end()
#define debug(v)                                                               \
  cout << #v << ":";                                                           \
  for (auto x : v) {                                                           \
    cout << x << ' ';                                                          \
  }                                                                            \
  cout << endl;
#define endl '\n'
int ctoi(const char c) {
  if ('0' <= c && c <= '9')
    return (c - '0');
  return -1;
}
ll gcd(ll a, ll b) { return (b == 0 ? a : gcd(b, a % b)); }
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
constexpr ll MOD = 1000000007;
constexpr ll INF = 1000000011;
constexpr ll MOD2 = 998244353;
constexpr ll LINF = 1001002003004005006ll;
constexpr ld EPS = 10e-8;
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &&x : v)
    is >> x;
  return is;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first;
  is >> p.second;
  return is;
}
template <typename T, typename U>
ostream &operator>>(ostream &os, const pair<T, U> &p) {
  os << p.first << ' ' << p.second;
  return os;
}
template <class T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (auto i = begin(v); i != end(v); ++i) {
    if (i != begin(v))
      os << ' ';
    os << *i;
  }
  return os;
}

int main() {
  int N, Q, com, A, B;
  string S;
  vector<set<int>> ST(26);
  cin >> N >> S >> Q;
  rep(i, N) {
    int X = S[i] - 'a';
    ST[X].insert(i);
  }
  rep(i, Q) {
    cin >> com;
    if (com == 1) {
      char C;
      cin >> A >> C;
      A--;
      int X = C - 'a';
      int Y = S[A] - 'a';
      ST[Y].erase(A);
      ST[X].insert(A);
      S[A] = C;
    } else {
      cin >> A >> B;
      A--;
      B--;
      int Z = 0;
      rep(i, 26) {
        if (lower_bound(ALL(ST[i]), A) != upper_bound(ALL(ST[i]), B)) {
          Z++;
        }
      }
      cout << Z << endl;
    }
  }
}

Fixed submission:
#include <algorithm>
#include <complex>
#include <cstdint>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <stdio.h>
#include <string>
#include <time.h>
#include <utility>
#include <vector>
using namespace std;
using pii = pair<int, int>;
using ll = long long;
using ld = long double;
#define pb push_back
#define mp make_pair
#define sc second
#define fr first
#define stpr setprecision
#define cYES cout << "YES" << endl
#define cNO cout << "NO" << endl
#define cYes cout << "Yes" << endl
#define cNo cout << "No" << endl
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define Rep(i, a, b) for (ll i = (a); i < (b); ++i)
#define rrep(i, n) for (ll i = n - 1; i >= 0; i--)
#define rRep(i, a, b) for (ll i = a; i >= b; i--)
#define crep(i) for (char i = 'a'; i <= 'z'; ++i)
#define psortsecond(A, N)                                                      \
  sort(A, A + N,                                                               \
       [](const pii &a, const pii &b) { return a.second < b.second; });
#define ALL(x) (x).begin(), (x).end()
#define debug(v)                                                               \
  cout << #v << ":";                                                           \
  for (auto x : v) {                                                           \
    cout << x << ' ';                                                          \
  }                                                                            \
  cout << endl;
#define endl '\n'
int ctoi(const char c) {
  if ('0' <= c && c <= '9')
    return (c - '0');
  return -1;
}
ll gcd(ll a, ll b) { return (b == 0 ? a : gcd(b, a % b)); }
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
constexpr ll MOD = 1000000007;
constexpr ll INF = 1000000011;
constexpr ll MOD2 = 998244353;
constexpr ll LINF = 1001002003004005006ll;
constexpr ld EPS = 10e-8;
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &&x : v)
    is >> x;
  return is;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first;
  is >> p.second;
  return is;
}
template <typename T, typename U>
ostream &operator>>(ostream &os, const pair<T, U> &p) {
  os << p.first << ' ' << p.second;
  return os;
}
template <class T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (auto i = begin(v); i != end(v); ++i) {
    if (i != begin(v))
      os << ' ';
    os << *i;
  }
  return os;
}

int main() {
  int N, Q, com, A, B;
  string S;
  vector<set<int>> ST(26);
  cin >> N >> S >> Q;
  rep(i, N) {
    int X = S[i] - 'a';
    ST[X].insert(i);
  }
  rep(i, Q) {
    cin >> com;
    if (com == 1) {
      char C;
      cin >> A >> C;
      A--;
      int X = C - 'a';
      int Y = S[A] - 'a';
      ST[Y].erase(A);
      ST[X].insert(A);
      S[A] = C;
    } else {
      cin >> A >> B;
      A--;
      B--;
      int Z = 0;
      rep(i, 26) {
        if (ST[i].lower_bound(A) != ST[i].upper_bound(B)) {
          Z++;
        }
      }
      cout << Z << endl;
    }
  }
}