Problem p02763 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define vtpl(x, y, z) vector<tuple<x, y, z>>
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const ll dx[8] = {0, -1, 0, 1, 1, -1, 1, -1};
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  ll n;
  cin >> n;
  string s;
  cin >> s;
  vector<set<ll>> v(26);
  rep(i, n) {
    ll a = s[i] - 'a';
    v[a].ins(i);
  }
  ll q;
  cin >> q;
  rep(i, q) {
    ll type;
    cin >> type;
    if (type == 1) {
      ll b;
      cin >> b;
      b--;
      char c;
      cin >> c;
      v[s[b] - 'a'].erase(b);
      v[c - 'a'].insert(b);
      s[b] = c;
    } else {
      ll b, c;
      cin >> b >> c;
      b--;
      c--;
      ll k = 0;
      rep(j, 26) {
        auto iter = lower_bound(all(v[j]), b);
        if (iter != v[j].end()) {
          if (*iter <= c)
            k++;
        }
      }
      cout << k << endl;
    }
  }
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define vtpl(x, y, z) vector<tuple<x, y, z>>
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const ll dx[8] = {0, -1, 0, 1, 1, -1, 1, -1};
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  ll n;
  cin >> n;
  string s;
  cin >> s;
  vector<set<ll>> v(26);
  rep(i, n) {
    ll a = s[i] - 'a';
    v[a].ins(i);
  }
  ll q;
  cin >> q;
  rep(i, q) {
    ll type;
    cin >> type;
    if (type == 1) {
      ll b;
      cin >> b;
      b--;
      char c;
      cin >> c;
      v[s[b] - 'a'].erase(b);
      v[c - 'a'].insert(b);
      s[b] = c;
    } else {
      ll b, c;
      cin >> b >> c;
      b--;
      c--;
      ll k = 0;
      rep(j, 26) {
        auto iter = v[j].lower_bound(b);
        if (iter != v[j].end()) {
          if (*iter <= c)
            k++;
        }
      }
      cout << k << endl;
    }
  }
  return 0;
}